Ultrathin MoS2 Nanosheets as Anode Materials for Sodium‐Ion Batteries with Superior Performance

Few‐layer MoS₂ nanosheets are successfully synthesized using a simple and scalable ultrasonic exfoliation technique. The thicknesses of the MoS₂ nanosheets ares about 10 nm as measured by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The unique nanosheet architecture renders the high‐rate transportation of sodium ions due to the short diffusion paths provided by ultrathin thickness and the large interlayer space within the MoS₂ crystal structure (d₍₀₀₂₎ = 6.38 Å). When applied as anode materials in sodium‐ion batteries, MoS₂ nanosheets exhibit a high, reversible sodium storage capacity and excellent cyclability. The MoS₂ nanosheets also demonstrate good electrochemical performance at high current densities.     

MoS2/Graphene Nanosheets from Commercial Bulky MoS2 and Graphite as Anode Materials for High Rate Sodium‐Ion Batteries

Tuning heterointerfaces between hybrid phases is a very promising strategy for designing advanced energy storage materials. Herein, a low‐cost, high‐yield, and scalable two‐step approach is reported to prepare a new type of hybrid material containing MoS₂/graphene nanosheets prepared from ball‐milling and exfoliation of commercial bulky MoS₂ and graphite. When tested as an anode material for a sodium‐ion battery, the as‐prepared MoS₂/graphene nanosheets exhibit remarkably high rate capability (284 mA h g^?1 at 20 A g^?1 (≈30C) and 201 mA h g^?1 at 50 A g^?1 (≈75C)) and excellent cycling stability (capacity retention of 95% after 250 cycles at 0.3 A g^?1). Detailed experimental measurements and density functional theory calculation reveal that the functional groups in 2D MoS₂/graphene heterostructures can be well tuned. The impressive rate capacity of the as‐prepared MoS₂/graphene hybrids should be attributed to the heterostructures with a low degree of defects and residual oxygen containing groups in graphene, which subsequently improve the electronic conductivity of graphene and decrease the Na⁺ diffusion barrier at the MoS₂/graphene interfaces in comparison with the acid treated one.     

1T′‐ReS2 Confined in 2D‐Honeycombed Carbon Nanosheets as New Anode Materials for High‐Performance Sodium‐Ion Batteries

ReS₂ (rhenium disulfide) is a new transition‐metal dichalcogenide that exhibits 1T′ phase and extremely weak interlayer van der Waals interactions. This makes it promising as an anode material for sodium‐ion batteries. However, achieving both a high‐rate capability and a long‐life has remained a major research challenge. Here, a new composite is reported, in which both are realized for the first time. 1T′‐ReS₂ is confined through strong interfacial interaction in a 2D‐honeycombed carbon nanosheets that comprise an rGO inter‐layer and a N‐doped carbon coating‐layer (rGO@ReS₂@N‐C). The strong interfacial interaction between carbon and ReS₂ increases overall conductivity and decreases Na⁺ diffusion resistance, whilst the intended 2D‐honeycombed carbon protective layer maintains structural morphology and electrochemical activity during long‐term cycling. These findings are confirmed by advanced characterization techniques, electrochemical measurement, and density functional theory calculation. The new rGO@ReS₂@N‐C exhibits the greatest rate performance reported so far for ReS₂ of 231 mAh g^?1 at 10 A g^?1. Significantly, this is together with ultra‐stable long‐term cycling of 192 mAh g^?1 at 2 A g^?1 after 4000 cycles.     

Defect‐Rich Soft Carbon Porous Nanosheets for Fast and High‐Capacity Sodium‐Ion Storage

Soft carbon has attracted tremendous attention as an anode in rocking‐chair batteries owing to its exceptional properties including low‐cost, tunable interlayer distance, and favorable electronic conductivity. However, it fails to exhibit decent performance for sodium‐ion storage owing to difficulties in the formation of sodium intercalation compounds. Here, microporous soft carbon nanosheets are developed via a microwave induced exfoliation strategy from a conventional soft carbon compound obtained by pyrolysis of 3,4,9,10‐perylene tetracarboxylic dianhydride. The micropores and defects at the edges synergistically leads to enhanced kinetics and extra sodium‐ion storage sites, which contribute to the capacity increase from 134 to 232 mAh g^?1 and a superior rate capability of 103 mAh g^?1 at 1000 mA g^?1 for sodium‐ion storage. In addition, the capacitance‐dominated sodium‐ion storage mechanism is identified through the kinetics analysis. The in situ X‐ray diffraction analyses are used to reveal that sodium ions intercalate into graphitic layers for the first time. Furthermore, the as‐prepared nanosheets can also function as an outstanding anode for potassium‐ion storage (reversible capacity of 291 mAh g^?1) and dual‐ion full cell (cell‐level capacity of 61 mAh g^?1 and average working voltage of 4.2 V). These properties represent the potential of soft carbon for achieving high‐energy, high‐rate, and low‐cost energy storage systems.     

2D Zn‐Hexamine Coordination Frameworks and Their Derived N‐Rich Porous Carbon Nanosheets for Ultrafast Sodium Storage

Metal‐organic coordination frameworks have been widely used as efficient precursors for the preparation of functional carbon‐based materials with various nanostructures. However, to date, the design of 2D carbon nanostructures from single coordination frameworks remains a great challenge. Herein, an efficient strategy for the fabrication of N‐rich porous carbon nanosheets from 2D Zn‐hexamine coordination framework nanosheets is developed. Remarkably, the N‐doping level of carbon nanosheets can attain 16.54 at%. In addition, the thickness of the carbon nanosheets can effectively be tuned by simply adjusting the molar ratio of the starting materials. As a proof‐of‐concept application, the as‐prepared carbon nanosheets as an anode material for sodium‐ion batteries exhibit an ultrafast sodium storage capability of 194 mAh g^?1 even at 10 A g^?1. As far as it is known, such a high‐rate capability has been rarely achieved in previous studies on carbonaceous anode materials for Na‐ion storage. Moreover, this approach is readily controllable and could be extended to prepare a series of 2D N‐doped carbon‐based nanomaterials on a large scale.     

Wet‐Chemical Processing of Phosphorus Composite Nanosheets for High‐Rate and High‐Capacity Lithium‐Ion Batteries

Phosphorus‐based materials are promising for high‐performance lithium‐ion battery (LIB) applications due to their high theoretical specific capacity. Currently, the existing physical methods render great difficulty toward rational engineering on the nanostructural phosphorus or its composites, thus limiting its high‐rate LIB applications. For the first time, a sublimation‐induced synthesis of phosphorus‐based composite nanosheets by a chemistry‐based solvothermal reaction is reported. Its formation mechanism involves solid–vapor–solid transformation driven by continuous vaporization–condensation process, as well as subsequent bottom‐up assembly growth. The proof‐of‐concept LIBs composed of the phosphorus‐based nanosheets achieve a high capacity of 630 mAh g^?1 at an ultrahigh current density of 20 A g^?1, which is attributed to efficient lithium‐ion diffusion and electron transfer. Such simple sublimation‐induced transformation opens up new prospects for rational engineering of phosphorus‐based materials for enhancing electrochemical performance.     

Liquid‐Phase Exfoliated Metallic Antimony Nanosheets toward High Volumetric Sodium Storage

Metallic antimony (Sb) with gray allotrope has rarely been considered from the viewpoint of two‐dimension layered system is actually a graphite‐like material, in which Sb layers consist of fused, ruffled, and six‐membered rings. Given that metallic Sb nanosheets can be played like graphene, it would be anticipated to obtain a new anode material with superior electrochemical performances for sodium storage. In this work, we propose an efficient strategy to fabricate free‐standing metallic Sb nanosheets via liquid‐phase exfoliation of gray Sb powder in an ios‐propyle alcohol (IPA) solution with a constant concentration of sodium hydroxide. As a proof of the concept, several hybrid films composed of metallic Sb nanosheets and graphene with tunable densities are achieved, in which the notorious volume change of metallic Sb can be efficiently alleviated with the aid of the good flexible graphene, and the whole density of electrode films can be significantly improved by harnessing the high density of Sb nanosheets. As a consequence, the optimized metallic Sb nanosheets‐graphene (SbNS‐G) film displays a high volumetric capacity of 1226 mAh cm^–3, high‐rate capability and good cycle performance for sodium storage.     

Spontaneous Growth of 3D Framework Carbon from Sodium Citrate for High Energy‐ and Power‐Density and Long‐Life Sodium‐Ion Hybrid Capacitors

Carbon sheets with 3D architectures, large graphitic interlayer spacing, and high electrical conductivity are highly expected to be an ideal anode material for sodium‐ion hybrid capacitors (SIHCs). Pursuing a simple synthesis methodology and advancing it from the laboratory to industry is of great importance. In this study, a new approach is presented to prepare 3D framework carbon (3DFC) with the above integrated advantages by a direct calcination of sodium citrate without aid of any additional carbon source, template, or catalyst. The first‐principle calculations verify that the large interlayer spacing and the curvature structure of 3DFC facilitate the sodium ion insertion/extraction. As a consequence, the optimal 3DFC sample exhibits high reversible capacity as well as excellent rate and cycling performance. On this basis, a dual‐carbon SIHC is fabricated by employing 3DFC as battery‐type anode and 3DFC‐derived nanoporous carbon as capacitor‐type cathode. It is able to deliver high energy‐ and power‐density feature as well as outstanding long‐term cycling stability in the potential range of 0–4.0 V. This study may open an avenue for developing high‐performance carbon electrode materials and pushes the practical applications of SIHCs a decisive step forward.     

Anodes and Sodium‐Free Cathodes in Sodium Ion Batteries

The increase in electricity generation poses growing demands on energy storage systems, thus offering a chance for the success of the reliable and cost‐effective energy storage technologies. Sodium ion batteries are emerging as such a technology, which is however not yet mature enough to enter the market. At the crux of building practical sodium ion batteries is the development of electrode materials that promise sufficient cost‐ and performance‐competitiveness. As such, herein, all typical sodium storage materials are discussed, considering their fabrication methods and sodiation mechanisms in detail. A comprehensive cross‐literature and cross‐material comparison, which also includes the related thermodynamic analysis of their sodiation products, is also provided. The review focusses particularly on anodes and sodium‐free cathodes, as they both play the role of the acceptor rather than the donor of sodium ions in their operation in batteries; their difference lies in the (de‐)sodiation voltage. In the discussion, special attention is paid to contradictory observations and interpretations in contemporary sodium ion battery research, since debates on these controversies are likely to fuel future sodium battery research.     

Confining Ultrathin 2D Superlattices in Mesoporous Hollow Spheres Renders Ultrafast and High‐Capacity Na‐Ion Storage

Sodium‐ion batteries have attracted ever‐increasing attention in view of the natural abundance of sodium resources. Sluggish sodiation kinetics, nevertheless, remain a tough challenge, in terms of achieving high rate capability and high energy density. Herein, a sheet‐in‐sphere nanoconfiguration of 2D titania–carbon superlattices vertically aligned inside of mesoporous TiO₂@C hollow nanospheres is constructed. In such a design, the ultrathin 2D superlattices consist of ordered alternating monolayers of titania and carbon, enabling interpenetrating pathways for rapid transport of electrons and Na⁺ ions as well as a 2D heterointerface for Na⁺ storage. Kinetics analysis discloses that the combination of 2D heterointerface and mesoporosity results an intercalation pseudocapacitive charge storage mechanism, which triggers ultrafast sodiation kinetics. In situ transmission electron microscope imaging and in situ synchrotron X‐ray diffraction techniques elucidate that the sheet‐in‐sphere architecture can maintain robust mechanical and crystallographic structural stability, resulting an extraordinary high rate capability, remarkable stable cycling with a low capacity fading ratio of 0.04% per cycle over 500 cycles at 0.2 C, and exceptionally long‐term cyclability up to 20 000 cycles at 50 C. This study offers a method for the realization of a high power density and long‐term cyclability battery by designing of a hierarchical nanoarchitecture.     

Fast Sodium Storage in TiO2@CNT@C Nanorods for High‐Performance Na‐Ion Capacitors

Na‐ion capacitors have attracted extensive interest due to the combination of the merits of high energy density of batteries and high power density as well as long cycle life of capacitors. Here, a novel Na‐ion capacitor, utilizing TiO₂@CNT@C nanorods as an intercalation‐type anode and biomass‐derived carbon with high surface area as an ion adsorption cathode in an organic electrolyte, is reported. The advanced architecture of TiO₂@CNT@C nanorods, prepared by electrospinning method, demonstrates excellent cyclic stability and outstanding rate capability in half cells. The contribution of extrinsic pseudocapacitance affects the rate capability to a large extent, which is identified by kinetics analysis. A key finding is that ion/electron transfer dynamics of TiO₂@CNT@C could be effectively enhanced due to the addition of multiwalled carbon nanotubes. Also, the biomass‐derived carbon with high surface area displays high specific capacity and excellent rate capability. Owing to the merits of structures and excellent performances of both anode and cathode materials, the assembled Na‐ion capacitors provide an exceptionally high energy density (81.2 W h kg^?1) and high power density (12 400 W kg^?1) within 1.0–4.0 V. Meanwhile, the Na‐ion capacitors achieve 85.3% capacity retention after 5000 cycles tested at 1 A g^?1.     

Ultrathin Li4Ti5O12 Nanosheet Based Hierarchical Microspheres for High‐Rate and Long‐Cycle Life Li‐Ion Batteries

Ultrathin Li₄Ti₅O₁₂ nanosheet based hierarchical microspheres are synthesized through a three‐step hydrothermal procedure. The average thickness of the Li₄Ti₅O₁₂ sheets is only ≈(6.6 ± 0.25) nm and the specific surface area of the sample is 178 m² g^?1. When applied into lithium ion batteries as anode materials, the hierarchical Li₄Ti₅O₁₂ microspheres exhibit high specific capacities at high rates (156 mA h g^?1 at 20 C, 150 mA h g^?1 at 50 C) and maintain a capacity of 126 mA h g^?1 after 3000 cycles at 20 C. The results clearly suggest that the utility of hierarchical structures based on ultrathin nanosheets can promote the lithium insertion/extraction reactions in Li₄Ti₅O₁₂. The obtained hierarchical Li₄Ti₅O₁₂ with ultrathin nanosheets and large specific surface area can be perfect anode materials for the lithium ion batteries applied in high power facilities, such as electric vehicles and hybrid electric vehicles.     

Rational Assembly of Hollow Microporous Carbon Spheres as P Hosts for Long‐Life Sodium‐Ion Batteries

This paper reports the rational assembly of novel hollow porous carbon nanospheres (HPCNSs) as the hosts of phosphorous (P) active materials for high‐performance sodium‐ion batteries (SIBs). The vaporization‐condensation process is employed to synthesize P/C composites, which is elucidated by both theories and experiments to achieve optimized designs. The combined molecular dynamics simulations and density functional theory calculations indicate that the morphologies of polymeric P₄ and the P loading in the P/C composites depend mainly on the pore size and surface condition of carbon supports. Micropores of 1–2 nm in diameter and oxygenated functional groups attached on carbon surface are essential for achieving high P loading and excellent structural stability. In light of these findings, HPCNS/amorphous red phosphorus composites with enhanced structural/functional features are synthesized, which present an extremely low volume expansion of ≈67.3% during cycles, much smaller than the commercial red P's theoretical value of ≈300%. The composite anodes deliver an exceptional sodium storage capacity and remarkable long‐life cyclic stability with capacity retention over 76% after 1000 cycles. This work signifies the importance of rational design of electrode materials based on accurate theoretical predictions and sheds light on future development of cost‐effective P/C composite anodes for commercially viable SIBs.     

Nanoarchitectured Array Electrodes for Rechargeable Lithium‐ and Sodium‐Ion Batteries

Rechargeable ion batteries have contributed immensely to shaping the modern world and been seriously considered for the efficient storage and utilization of intermittent renewable energies. To fulfill their potential in the future market, superior battery performance of high capacity, great rate capability, and long lifespan is undoubtedly required. In the past decade, along with discovering new electrode materials, the focus has been shifting more and more toward rational electrode designs because the performance is intimately connected to the electrode architectures, particularly their designs at the nanoscale that can alleviate the reliance on the materials' intrinsic nature. The utilization of nanoarchitectured arrays in the design of electrodes has been proven to significantly improve the battery performance. A comprehensive summary of the structural features and fabrications of the nanoarchitectured array electrodes is provided, and some of the latest achievements in the area of both lithium‐ and sodium‐ion batteries are highlighted. Finally, future challenges and opportunities that would allow further development of such advanced electrode configuration are discussed.     

CuS Microspheres with Tunable Interlayer Space and Micropore as a High‐Rate and Long‐Life Anode for Sodium‐Ion Batteries

Layered transition metal sulfides (LTMSs) have tremendous commercial potential in anode materials for sodium‐ion batteries (SIBs) in large‐scale energy storage application. However, it is a great challenge for most LTMS electrodes to have long cycling life and high‐rate capability due to their larger volume expansion and the formation of soluble polysulfide intermediates caused by the conversion reaction. Herein, layered CuS microspheres with tunable interlayer space and pore volumes are reported through a cost‐effective interaction method using a cationic surfactant of cetyltrimethyl ammonium bromide (CTAB). The CuS–CTAB microsphere as an anode for SIBs reveals a high reversible capacity of 684.6 mAh g^?1 at 0.1 A g^?1, and 312.5 mAh g^?1 at 10 A g^?1 after 1000 cycles with high capacity retention of 90.6%. The excellent electrochemical performance is attributed to the unique structure of this material, and a high pseudocapacitive contribution ensures its high‐rate performance. Moreover, in situ X‐ray diffraction is applied to investigate their sodium storage mechanism. It is found that the long chain CTAB in the CuS provides buffer space, traps polysulfides, and restrains the further growth of Cu particles during the conversion reaction process that ensure the long cycling stability and high reversibility of the electrode material.     

Sodium Storage and Transport Properties in Layered Na2Ti3O7 for Room‐Temperature Sodium‐Ion Batteries

Layered sodium titanium oxide, Na₂Ti₃O₇, is synthesized by a solid‐state reaction method as a potential anode for sodium‐ion batteries. Through optimization of the electrolyte and binder, the microsized Na₂Ti₃O₇ electrode delivers a reversible capacity of 188 mA h g^?1 in 1 M NaFSI/PC electrolyte at a current rate of 0.1C in a voltage range of 0.0–3.0 V, with sodium alginate as binder. The average Na storage voltage plateau is found at ca. 0.3 V vs. Na⁺/Na, in good agreement with a first‐principles prediction of 0.35 V. The Na storage properties in Na₂Ti₃O₇ are investigated from thermodynamic and kinetic aspects. By reducing particle size, the nanosized Na₂Ti₃O₇ exhibits much higher capacity, but still with unsatisfied cyclic properties. The solid‐state interphase layer on Na₂Ti₃O₇ electrode is analyzed. A zero‐current overpotential related to thermodynamic factors is observed for both nano‐ and microsized Na₂Ti₃O₇. The electronic structure, Na⁺ ion transport and conductivity are investigated by the combination of first‐principles calculation and electrochemical characterizations. On the basis of the vacancy‐hopping mechanism, a quasi‐3D energy favorable trajectory is proposed for Na₂Ti₃O₇. The Na⁺ ions diffuse between the TiO₆ octahedron layers with pretty low activation energy of 0.186 eV.     

Two‐Dimensional Materials for Beyond‐Lithium‐Ion Batteries

Lithium‐ion batteries (LIBs) have dominated the portable electronics industry and solid‐state electrochemical research and development for the past two decades. In light of possible concerns over the cost and future availability of lithium, sodium‐ion batteries (SIBs) and other new technologies have emerged as candidates for large‐scale stationary energy storage. Research in these technologies has increased dramatically with a focus on the development of new materials for both the positive and negative electrodes that can enhance the cycling stability, rate capability, and energy density. Two‐dimensional (2D) materials are showing promise for many energy‐related applications and particularly for energy storage, because of the efficient ion transport between the layers and the large surface areas available for improved ion adsorption and faster surface redox reactions. Recent research highlights on the use of 2D materials in these future ‘beyond‐lithium‐ion’ battery systems are reviewed, and strategies to address challenges are discussed as well as their prospects.