Anomalous interfacial stress generation during sodium intercalation/extraction in MoS2 thin-film anodes

Heterogeneous MoS2 Nanosheets on Porous TiO2 Nanofibers toward Fast and Reversible Sodium-Ion Storage

2D layered molybdenum disulfide (MoS2) has garnered considerable attention as an attractive electrode material in sodium-ion batteries (SIBs), but sluggish mass transfer kinetic and capacity fading make it suffer from inferior cycle capability. Herein, hierarchical MoS2 nanosheets decorated porous TiO2 nanofibers (MoS2 NSs@TiO2 NFs) with rich oxygen vacancies are engineered by microemulsion electrospinning method and subsequent hydrothermal/heat treatment. The MoS2 NSs@TiO2 NFs improves ion/electron transport kinetic and long-term cycling performance through distinctive porous structure and heterogeneous component. Consequently, the electrode exhibits excellent long-term Na storage capacity (298.4 mAh g-1 at 5 A g-1 over 1100 cycles and 235.6 mAh g-1 at 10 A g-1 over 7200 cycles). Employing Na3V2(PO4)3 as cathode, the full cell maintains a desirable capacity of 269.6 mAh g-1 over 700 cycles at 1.0 A g-1. The stepwise intercalation-conversion and insertion/extraction endows outstanding Na+ storage performance, which yields valuable insight into the advancement of fast-charging and long-cycle life SIBs anode materials.

Unlocking a Fast Track for Magnesium Migration in Cu1.8S1-xSex via Anion-Tuning Chemistry

Rechargeable magnesium batteries (rMBs) are promising candidates for next-generation batteries in which sulfides are widely used as cathode materials. The slow kinetics, low redox reversibility, and poor magnesium storage stability induced by the large Coulombic resistance and ionic polarization of Mg2+ ions have obstructed the development of high-performance rMBs. Herein, a Cu1.8S1-xSex cathode material with a two-dimensional sheet structure has been prepared by an anion-tuning strategy, achieving improved magnesium storage capacity and cycling stability. Element-specific synchrotron radiation analysis is evidence that selenium incorporation has indeed changed the chemical state of Cu species. Density functional theory calculations combined with kinetics analysis reveal that the anionic substitution endows the Cu1.8S1-xSex electrode with favorable charge-transfer kinetics and low ion diffusion barrier. The principal magnesium storage mechanisms and structural evolution process have been revealed in details based on a series of ex situ investigations. Our findings provide an effective heteroatom-tuning tactic of optimizing electrode structure toward advanced energy storage devices.

Functionalized MBenes as promising anode materials for high-performance alkali-ion batteries: a first-principles study

The discovery of novel electrode materials based on two-dimensional (2D) structures is critical for alkali metal-ion batteries. Herein, we performed first-principles computations to investigate functionalized MXenes, Mo2BT2(T = O, S), which are also regarded as B-based MXenes, or named as MBenes, as potential anode materials for Li-ion batteries and beyond. The pristine and T-terminated Mo2BT2(T = O, S) monolayers reveal metallic character with higher electronic conductivity and are thermodynamically stable with an intrinsic dipole moment. Both Mo2BO2and Mo2BS2monolayers exhibit high theoretical Li/Na/K storage capacity and low ion diffusion barriers. These findings suggest that functionalized Mo2BT2(T = O, S) monolayers are promising for designing viable anode materials for high-performance alkali-ion batteries.

Prediction of boridenes as high-performance anodes for alkaline metal and alkaline Earth metal ion batteries

We conducted a comprehensive density functional theory investigation using the r²SCAN-rVV10 functional on the structural stability and electrochemical properties of boridenes for their use as anode materials in rechargeable alkaline (earth) metal-ion batteries (Li⁺, Na⁺, K⁺, Mg²⁺ and Ca²⁺). According to first-principles molecular dynamics simulations and reaction thermodynamic calculations, Mo_4/3B₂(OH)₂ and Mo_4/3B₂F₂ are unstable in the presence of alkaline (earth) metal ions due to the surface-conversion reactions between the surface terminations and adsorbates. Meanwhile, the bare Mo_4/3B₂ and Mo_4/3B₂O₂ monolayers not only can accommodate alkaline (earth) metal ions, but also form stable multi-layer adsorption structures for most of the studied metal ions (Li⁺, Na⁺, K⁺, Mg²⁺ and Ca²⁺). The predicted gravimetric capacities of the bare Mo_4/3B₂ monolayer (Mo_4/3B₂O₂) are 625.9 mA h g^-1 (357.3 mA h g^-1), 247.20 mA h g^-1 (392.1 mA h g^-1), 101.8 mA h g^-1 (206.4 mA h g^-1), 667.0 mA h g^-1, and 413.0 mA h g^-1 (485.4 mA h g^-1) for Li⁺, Na⁺, K⁺, Mg²⁺ and Ca²⁺ ions, respectively. The bare Mo_4/3B₂ exhibits lower onset charging open circuit voltages for alkaline (earth) metal ions than that of Mo_4/3B₂O₂. The diffusivities of the metal ions were revealed to be high on the boridene monolayer especially for the outer fully stable adsorption layers, where the migration energy barriers were found to be less than 0.10 eV. Similar to that of MXenes, the negative electron cloud (NEC) also plays a vital role in stabilizing the observed multi-layer adsorption structures for various metal ions on either the bare Mo_4/3B₂ or Mo_4/3B₂O₂ monolayer.

Elucidating the Synergic Effect in Nanoscale MoS2 /TiO2 Heterointerface for Na-Ion Storage

Interface engineering in electrode materials is an attractive strategy for enhancing charge storage, enabling fast kinetics, and improving cycling stability for energy storage systems. Nevertheless, the performance improvement is usually ambiguously ascribed to the "synergetic effect", the fundamental understanding toward the effect of the interface at molecular level in composite materials remains elusive. In this work, a well-defined nanoscale MoS₂ /TiO₂ interface is rationally designed by immobilizing TiO₂ nanocrystals on MoS₂ nanosheets. The role of heterostructure interface between TiO₂ and MoS₂ by operando synchrotron X-ray diffraction (sXRD), solid-state nuclear magnetic resonance, and density functional theory calculations is investigated. It is found that the existence of a hetero-interfacial electric field can promote charge transfer kinetics. Based on operando sXRD, it is revealed that the heterostructure follows a solid-solution reaction mechanism with small volume changes during cycling. As such, the electrode demonstrates ultrafast Na⁺ ions storage of 300 mAh g^-1 at 10 A g^-1 and excellent reversible capacity of 540 mAh g^-1 at 0.2 A g^-1 . This work provides significant insights into understanding of heterostructure interface at molecular level, which suggests new strategies for creating unconventional nanocomposite electrode materials for energy storage systems.

X-Ray Spectromicroscopy Investigation of Heterogeneous Sodiation in Hard Carbon Nanosheets with Vertically Oriented (002) Planes

Hard carbon (HC) is a promising anode material for sodium-ion batteries, but the performance remains unsatisfactory and the sodiation mechanism in HC is one of the most debated topics. Here, from self-assembled cellulose nanocrystal sheets with crystallographic texture, unique HC nanosheets with vertically oriented (002) planes are fabricated and used as a model HC to investigate the sodiation mechanisms using synchrotron scanning transmission X-ray microscopy (STXM) coupled with analytical transmission electron microscopy (TEM). The model HC simplifies the 3D sodiation in a typical HC particle into a 2D sodiation, which facilitates the visualization of phase transformation at different states of charge. The results for the first time unveil that the sodiation in HC initiates heterogeneously, with multiple propagation fronts proceeding simultaneously, eventually merging into larger aggregates. The spatial correlation between the preferential adsorption and nucleation sites suggests that the heterogeneous nucleation is driven by the local Na-ion concentration, which is determined by defects or heteroatoms that have strong binding to Na ions. By identifying intercalation as the dominant sodium storage mechanism in the model HC, the findings highlight the importance of engineering the graphene layer orientation and the structural heterogeneity of edge sites to enhance the performances.

Updated Insights into 3D Architecture Electrodes for Micropower Sources

Microbatteries (MBs) and microsupercapacitors (MSCs) are primary on-chip micropower sources that drive autonomous and stand-alone microelectronic devices for implementation of the Internet of Things (IoT). However, the performance of conventional MBs and MSCs is restricted by their 2D thin-film electrode design, and these devices struggle to satisfy the increasing IoT energy demands for high energy density, high power density, and long lifespan. The energy densities of MBs and MSCs can be improved significantly through adoption of a 2D thick-film electrode design; however, their power densities and lifespans deteriorate with increased electrode thickness. In contrast, 3D architecture electrodes offer remarkable opportunities to simultaneously improve MB and MSC energy density, power density, and lifespan. To date, various 3D architecture electrodes have been designed, fabricated, and investigated for MBs and MSCs. This review provides an update on the principal superiorities of 3D architecture electrodes over 2D thick-film electrodes in the context of improved MB and MSC energy density, power density, and lifespan. In addition, the most recent and representative progress in 3D architecture electrode development for MBs and MSCs is highlighted. Finally, present challenges are discussed and key perspectives for future research in this field are outlined.

1T MoS2growth from exfoliated MoS2nucleation as high rate anode for sodium storage

Recently, metallic 1T MoS₂has been investigated due to its excellent performance in electrocatalysts, photocatalysts, supercapacitors and secondary batteries. However, there are only a few fabrication methods to synthesize stable 1T MoS₂. In this work, exfoliated MoS₂is employed as seed crystals for the nucleation and growth of a stable 1T MoS₂grains by an epitaxial growth strategy. The 1T MoS₂displays a large interlayer spacing around 0.95 nm, excellent hydrophilia and more electrochemically active sites along the basal plane, which contribute an efficient ion/electron transport pathway and structural stability. When employed as the anode material for sodium ion batteries, the 1T MoS₂electrodes can survive 500 full charge/discharge cycles with a minimum capacity loss of 0.40 mAh g^-1cycle^-1tested at a current density of 1.0 A g^-1, and the capacity degradation is as low as 0.39 mAh g^-1cycle^-1at a current density of 2.0 A g^-1.

Revealing molecular conformation-induced stress at embedded interfaces of organic optoelectronic devices by sum frequency generation spectroscopy

Interface stresses are pervasive and critical in conventional optoelectronic devices and generally lead to many failures and reliability problems. However, detection of the interface stress embedded in organic optoelectronic devices is a long-standing problem, which causes the unknown relationship between interface stress and organic device stability (one key and unsettled issue for practical applications). In this study, a kind of previously unknown molecular conformation-induced stress is revealed at the organic embedded interface through sum frequency generation (SFG) spectroscopy technique. This stress can be greater than 10 kcal/mol per nm² and is sufficient to induce molecular disorder in the organic semiconductor layer (with energy below 8 kcal/mol per nm²), finally causing instability of the organic transistor. This study not only reveals interface stress in organic devices but also correlates instability of organic devices with the interface stress for the first time, offering an effective solution for improving device stability.

Layer-by-Layer Self-Assembled Nanostructured Electrodes for Lithium-Ion Batteries

Gaining control over the nanoscale assembly of different electrode components in energy storage systems can open the door for design and fabrication of new electrode and device architectures that are not currently feasible. This work presents aqueous layer-by-layer (LbL) self-assembly as a route towards design and fabrication of advanced lithium-ion batteries (LIBs) with unprecedented control over the structure of the electrode at the nanoscale, and with possibilities for various new designs of batteries beyond the conventional planar systems. LbL self-assembly is a greener fabrication route utilizing aqueous dispersions that allow various Li⁺ intercalating materials assembled in complex 3D porous substrates. The spatial precision of positioning of the electrode components, including ion intercalating phase and electron-conducting phase, is down to nanometer resolution. This capable approach makes a lithium titanate anode delivering a specific capacity of 167 mAh g^-1 at 0.1C and having comparable performances to conventional slurry-cast electrodes at current densities up to 100C. It also enables high flexibility in the design and fabrication of the electrodes where various advanced multilayered nanostructures can be tailored for optimal electrode performance by choosing cationic polyelectrolytes with different molecular sizes. A full-cell LIB with excellent mechanical resilience is built on porous insulating foams.

A multi-layered composite assembly of Bi nanospheres anchored on nitrogen-doped carbon nanosheets for ultrastable sodium storage

Bismuth (Bi) is a promising anode candidate for sodium ion batteries (SIBs) with a high volumetric capacity (3765 mA h cm-3) and moderate working potential but suffers from large volume change (ca. 250%) during the sodiation/desodiation process, resulting in pulverization of the electrode, electrical contact loss, excessive accumulation of solid electrolyte interfaces, etc., devastating the cycling stability of the electrode seriously. Addressing this issue significantly relies on rational micro- and nano-structuring. Herein, we prepared a 3D multi-layered composite assembly of Bi/carbon heterojunctions with 0D bismuth nanospheres distributed and anchored on 2D nitrogen-doped carbon nanosheets (NCSs), using a preorganization strategy by taking full advantage of the strong complexation ability of Bi3+. The multi-layered composite assembly is periodic and close-packed, with Bi nanospheres <25 nm, carbon nanosheets ∼30 nm, and an average interlayer space of ∼75 nm. Such a specific architecture provides abundant electrochemically active surfaces and ion migration channels as the Bi nanospheres are attached to the 2D nitrogen-doped carbon nanosheets via a point-to-surface pattern. Moreover, the mono-layer Bi nanospheres oriented along the 2D-surface of NCSs are kinetically favorable for the recognition of Na+ by the active sites of Bi nanospheres as well as for avoiding the long distance migration of Na+ (external diffusion of Na+). Furthermore, thermodynamically, the small size and high surface energy of ultrasmall Bi nanospheres could contribute to high ion mobility (internal diffusion of Na+) and promote electrochemical reactions as well. The multi-layered composite assembly of Bi@NCSs (ML-Bi@NCSs) not only provides a robust 3D framework guaranteeing the whole structural stability but also ensures direct and full contact of each active nano-building block with electrolyte, thereby forming a high-throughput electron/ion transport system. When evaluated as the anode for SIBs, ML-Bi@NCSs deliver superior high-rate capability up to 30 A g-1 (specific capacity: 288 mA h g-1) and long-term cycling stability (capacity retention: 95.8% after 5000 cycles at 10 A g-1 and 90.6% after 10 000 cycles at 20 A g-1, respectively).

2D Sandwiched Nano Heterostructures Endow MoSe2 /TiO2- x /Graphene with High Rate and Durability for Sodium Ion Capacitor and Its Solid Electrolyte Interphase Dependent Sodiation/Desodiation Mechanism

Nano heterostructures relying on their versatile construction and the breadth of combined functionality have shown great potential in energy storage fields. Herein, 2D sandwiched MoSe₂ /TiO_{2- x} /graphene nano heterostructures are designed by integrating structural and functional effects of each component, aiming to address the rate capability and cyclic stability of MoSe₂ for sodium ion capacitors (SICs). These 2D nano heterostructures based on graphene platform can facilitate the interfacial electron transport, giving rise to fast reaction kinetics. Meanwhile, the 2D open structure induces a large extent of surface capacitive contribution, eventually leading to a high rate capability (415.2 mAh g^-1 @ 5 A g^-1 ). An ultrathin oxygen deficient TiO_{2- x} layer sandwiched in these nano heterostructures provides a strong chemical-anchoring regarding the products generated during the sodiation/desodiation process, securing the entire cyclic stability. The associated sodiation/desodiation mechanism is revealed by operando and ex situ characterizations, which exhibits a strong solid electrolyte interphase (SEI) dependence. The simulations verify the dependent sodiation products and enhanced heterostructural chemical-anchoring. Assembled SICs based on these nano heterostructures anode exhibit high initial Coulombic efficiency, energy/power densities, and long cycle life, shedding new light on the design of nano heterostructure electrodes for high performance energy storage application.

Exploring the structure evolution of MoS2 upon Li/Na/K ion insertion and the origin of the unusual stability in potassium ion batteries

The recent revival of research on Na and K ion batteries has two benefits. It not only provides alternate energy storage technologies to Li ion batteries with potential cost advantages but also enhances our understanding of charge storage through systematic studies on alkali-metal ion batteries with increasing insertion ion sizes. Using MoS2 as a model material, the structure evolution upon the uptake of Li, Na, and K ions are compared through in situ TEM. Despite their larger size, insertion of K ions shows both the better electrochemical and structural stability. To understand this paradoxical and counter-intuitive phenomenon, in situ XRD is carried out to examine the phase transitions of MoS2 upon ion insertion, while ex situ TEM is further applied to closely examine the structures at the nanoscale. Complementary DFT calculations are performed to understand the kinetic/thermodynamic origins of the unusual stability. The result reveal that the less electrovalent K-S bond favors the intercalation process, resulting in preservation of the layered structure for stable cycling. This study provides a structural insight to design stable electrodes for the K-ion batteries.

A boron-exposed TiB3 monolayer with a lower electrostatic-potential surface as a higher-performance anode material for Li-ion and Na-ion batteries

The lack of high-performance anode materials has become a major obstacle to the development of Li- and Na-ion batteries. Recently, 2D transition metal borides (e.g. MBenes) have attracted much attention due to their excellent stability and electrical conductivity. Unfortunately, most of the reported MBene phases typically have an intrinsic metal-rich structure with metal atoms exposed on the surface, which harmfully affect the adsorption of Li/Na atoms. Here, through crystal structure prediction combined with the first-principles density functional theory, a novel TiB3 MBene has been determined by altering the proportion of non-metallic element boron to wrap metal atoms and weaken nearest-neighbor electrostatic repulsion. Electrostatic potential analysis visually shows a surface with low potential on the TiB3 monolayer implying high adsorption capacity, and also can be used to quickly screen out the Li/Na adsorption sites. Accurate half-cell battery simulation confirmably shows that the TiB3 monolayer possesses a theoretical specific capacity of 1335.04 and 667.52 mA h g-1 for Li and Na, respectively. The TiB3 monolayer can remain metallic after adsorbing Li/Na atoms, which ensures good conductivity during battery cycling. The ultra-low barrier energy (only 38 meV for Li) and suitable open-circuit voltage indicate excellent charging and discharging capabilities. These results suggest that the TiB3 monolayer could be a promising anode material for Li- and Na-ion batteries, and provide a simple design principle for exposing non-metallic atoms on the surface.

Recent Advances on Mixed Metal Sulfides for Advanced Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have drawn enormous attention in the past few years from both academic and industrial battery communities in view of the fascinating advantages of rich abundance and low cost of sodium resources. Among various electrode materials, mixed metal sulfides (MMSs) stand out as promising negative electrode materials for SIBs considering their superior structural and compositional advantages, such as decent electrochemical reversibility, high electronic conductivity, and rich redox reactions. Here, a summary of some recent developments in the rational design and synthesis of various kinds of MMSs with tailorable architectures, structural/compositional complexity, controllable morphologies, and enhanced electrochemical properties is presented. The effect of structural engineering and compositional design of MMSs on the sodium storage properties is highlighted. It is anticipated that further innovative works on the material design of advanced electrodes for batteries can be inspired.

Rationally Designed Three-Layered Cu2 S@Carbon@MoS2 Hierarchical Nanoboxes for Efficient Sodium Storage

Hybrid materials, integrating the merits of individual components, are ideal structures for efficient sodium storage. However, the construction of hybrid structures with decent physical/electrochemical properties is still challenging. Now, the elaborate design and synthesis of hierarchical nanoboxes composed of three-layered Cu₂ S@carbon@MoS₂ as anode materials for sodium-ion batteries is reported. Through a facile multistep template-engaged strategy, ultrathin MoS₂ nanosheets are grown on nitrogen-doped carbon-coated Cu₂ S nanoboxes to realize the Cu₂ S@carbon@MoS₂ configuration. The design shortens the diffusion path of electrons/Na⁺ ions, accommodates the volume change of electrodes during cycling, enhances the electric conductivity of the hybrids, and offers abundant active sites for sodium uptake. By virtue of these advantages, these three-layered Cu₂ S@carbon@MoS₂ hierarchical nanoboxes show excellent electrochemical properties in terms of decent rate capability and stable cycle life.

In situ chemically encapsulated and controlled SnS2 nanocrystal composites for durable lithium/sodium-ion batteries

SnS2 as the promising anode for lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) still encounters the undesirable rate performance and cycle stability. Herein, a unique stable structure is developed, where the SnS2 nanocrystals (NCs) are sturdily encapsulated by carbon shells anchored on a reduced graphene oxide (rGO) via the one-pot solvothermal process. The well-controlled carbon shells provide the enduring protection for SnS2 NCs through C-S covalent bonds from the corrosion of electrolyte and pulverization of structure. Moreover, both experimental results and density functional theory (DFT) calculations demonstrate that the carbon protective shell effectively enhances the structure stability and conductivity of the resulting materials. Interestingly, the size of SnS2 NCs and the thickness of carbon shells are accurately controlled by regulating the content of glucose. Aided by the advanced electron/ion transfer kinetics and structure stability, the SnS2-based electrode exhibits desired lithium/sodium storage performance and unprecedented long-term cycling stability (capacity retention of 74.7% after 1000 cycles at 2 A g-1 for LIBs and 102% after 200 cycles at 500 mA g-1 for SIBs). This work develops a method for promoting the practical applications and large-scale production of SnS2 composites for energy storage devices.

Hierarchical Nanostructured NiS/MoS2/C Composite Hollow Spheres for High Performance Sodium-Ion Storage Performance

Development of high performance electrode materials for sodium-ion batteries has been given a lot of attention in recent years but challenges remain. Herein, we have successfully synthesized the first NiS/MoS₂/C composite hollow spheres (NMSCHSs) via a facile hard template method combined with calcined sulfidation process. Owing to its unique hierarchical nanostructure with NiS nanoparticles embedded in shell wrapped with MoS₂ nanosheets, the NMSCHS-based anode electrodes exhibit superior rate capability and cycling stability, with a specific discharge capacity of 516 mAh g^-1 and 98.5% retention after 60 cycles at a current density of 0.1 A g^-1 and a discharge capacity of 398 mAh g^-1 even at 5 A g^-1.

Interfacial Residual Stress Relaxation in Perovskite Solar Cells with Improved Stability

To improve the photovoltaic performance (both efficiency and stability) in hybrid organic-inorganic halide perovskite solar cells, perovskite lattice distortion is investigated with regards to residual stress (and strain) in the polycrystalline thin films. It is revealed that residual stress is concentrated at the surface of the as-prepared film, and an efficient method is further developed to release this interfacial stress by A site cation alloying. This results in lattice reconstruction at the surface of polycrystalline thin films, which in turn results in low elastic modulus. Thus, a "bone-joint" configuration is constructed within the interface between the absorber and the carrier transport layer, which improves device performance substantially. The resultant photovoltaic devices exhibit an efficiency of 21.48% with good humidity stability and improved resistance against thermal cycling.

SnS2 /Co3 S4 Hollow Nanocubes Anchored on S-Doped Graphene for Ultrafast and Stable Na-Ion Storage

SnS₂ has been widely studied as an anode material for sodium-ion batteries (SIBs) based on the high theoretical capacity and layered structure. Unfortunately, rapid capacity decay associated with volume variation during cycling limits practical application. Herein, SnS₂ /Co₃ S₄ hollow nanocubes anchored on S-doped graphene are synthesized for the first time via coprecipitation and hydrothermal methods. When applied as the anode for SIBs, the sample delivers a distinguished charge specific capacity of 1141.8 mAh g^-1 and there is no significant capacity decay (0.1 A g^-1 for 50 cycles). When the rate is increased to 0.5 A g^-1 , it presents 845.7 mAh g^-1 after cycling 100 times. Furthermore, the composite also exhibits an ultrafast sodium storage capability where 392.9 mAh g^-1 can be obtained at 10 A g^-1 and the charging time is less than 3 min. The outstanding electrochemical properties can be ascribed to the enhancement of conductivity for the addition of S-doped graphene and the existence of p-n junctions in the SnS₂ /Co₃ S₄ heterostructure. Moreover, the presence of mesopores between nanosheets can alleviate volume expansion during cycling as well as being beneficial for the migration of Na⁺ .