Conversion‐Type MnO Nanorods as a Surprisingly Stable Anode Framework for Sodium‐Ion Batteries

The emergence of nanomaterials in the past decades has greatly advanced modern energy storage devices. Nanomaterials can offer high capacity and fast kinetics yet are prone to rapid morphological evolution and degradation. As a result, they are often hybridized with a stable framework in order to gain stability and fully utilize its advantages. However, candidates for such framework materials are rather limited, with carbon, conductive polymers, and Ti‐based oxides being the only choices; note these are all inactive or intercalation compounds. Conventionally, alloying‐/conversion‐type electrodes, which are thought to be electrochemically unstable by themselves, have never been considered as framework materials. This concept is challenged. Successful application of conversion‐type MnO nanorod as a anode framework for high‐capacity Mo₂C/MoO_x nanoparticles has been demonstrated in sodium‐ion batteries. Surprisingly, it can stably deliver 110 mAh g^?1 under extremely high rate of 8000 mA g^?1 (≈70 C) over 40 000 cycles with no capacity decay. More generally, this is considered as a proof of concept and much more alloying‐/conversion‐type materials are expected to be explored for such applications.     

Fluorinated Polyimide as a Novel High‐Voltage Binder for High‐Capacity Cathode of Lithium‐Ion Batteries

An increase in the energy density of lithium‐ion batteries has long been a competitive advantage for advanced wireless devices and long‐driving electric vehicles. Li‐rich layered oxide, xLi₂MnO₃?(1?x)LiMn_1?y?zNi_yCo_zO₂, is a promising high‐capacity cathode material for high‐energy batteries, whose capacity increases by increasing charge voltage to above 4.6 V versus Li. Li‐rich layered oxide cathode however suffers from a rapid capacity fade during the high‐voltage cycling because of instable cathode–electrolyte interface, and the occurrence of metal dissolution, particle cracking, and structural degradation, particularly, at elevated temperatures. Herein, this study reports the development of fluorinated polyimide as a novel high‐voltage binder, which mitigates the cathode degradation problems through superior binding ability to conventional polyvinylidenefluoride binder and the formation of robust surface structure at the cathode. A full‐cell consisting of fluorinated polyimide binder‐assisted Li‐rich layered oxide cathode and conventional electrolyte without any electrolyte additive exhibits significantly improved capacity retention to 89% at the 100th cycle and discharge capacity to 223–198 mA h g^?1 even under the harsh condition of 55 °C and high charge voltage of 4.7 V, in contrast to a rapid performance fade of the cathode coated with polyvinylidenefluoride binder.     

Applications of Tin Sulfide‐Based Materials in Lithium‐Ion Batteries and Sodium‐Ion Batteries

SnS_x (x = 1, 2) compounds are composed of earth‐abundant elements and are nontoxic and low‐cost materials that have received increasing attention as energy materials over the past decades, owing to their huge potential in batteries. Generally, SnS_x materials have excellent chemical stability and high theoretical capacity and reversibility due to their unique 2D‐layered structure and semiconductor properties. As a promising matrix material for storing different alkali metal ions through alloying/dealloying reactions, SnS_x compounds have broad electrochemical prospects in batteries. Herein, the structural properties of SnS_x materials and their advantages as electrode materials are discussed. Furthermore, detailed accounts of various synthesis methods and applications of SnS_x materials in lithium‐ion batteries, sodium‐ion batteries, and other new rechargeable batteries are emphasized. Ultimately, the challenges and opportunities for future research on SnS_x compounds are discussed based on the available academic knowledge, including recent scientific advances.     

Two‐Dimensional Molybdenum Trioxide and Dichalcogenides

In the quest to discover the properties of planar semiconductors, two‐dimensional molybdenum trioxide and dichalcogenides have recently attracted a large amount of interest. This family, which includes molybdenum trioxide (MoO₃), disulphide (MoS₂), diselenide (MoSe₂) and ditelluride (MoTe₂), possesses many unique properties that make its compounds appealing for a wide range of applications. These properties can be thickness dependent and may be manipulated via a large number of physical and chemical processes. In this Feature Article, a comprehensive review is delivered of the fundamental properties, synthesis techniques and applications of layered and planar MoO₃, MoS₂, MoSe₂, and MoTe₂ along with their future prospects.     

Approaching the Downsizing Limit of Maricite NaFePO4 toward High‐Performance Cathode for Sodium‐Ion Batteries

Maricite NaFePO₄ nanodots with minimized sizes (≈1.6 nm) uniformly embedded in porous N‐doped carbon nanofibers (designated as NaFePO₄@C) are first prepared by electrospinning for maximized Na‐storage performance. The obtained flexible NaFePO₄@C fiber membrane adherent on aluminum foil is directly used as binder‐free cathode for sodium‐ion batteries, revealing that the ultrasmall nanosize effect as well as a high‐potential desodiation process can transform the generally perceived electrochemically inactive maricite NaFePO₄ into a highly active amorphous phase; meanwhile, remarkable electrochemical performance in terms of high reversible capacity (145 mA h g^?1 at 0.2 C), high rate capability (61 mA h g^?1 at 50 C), and unprecedentedly high cyclic stability (≈89% capacity retention over 6300 cycles) is achieved. Furthermore, the soft package Na‐ion full battery constructed by the NaFePO₄@C nanofibers cathode and the pure carbon nanofibers anode displays a promising energy density of 168.1 Wh kg^?1 and a notable capacity retention of 87% after 200 cycles. The distinctive 3D network structure of very fine NaFePO₄ nanoparticles homogeneously encapsulated in interconnected porous N‐doped carbon nanofibers, can effectively improve the active materials' utilization rate, facilitate the electrons/Na⁺ ions transport, and strengthen the electrode stability upon prolonged cycling, leading to the fascinating Na‐storage performance.     

Urchin‐Like CoSe2 as a High‐Performance Anode Material for Sodium‐Ion Batteries

Urchin‐like CoSe₂ assembled by nanorods has been synthesized via simple solvothermal route and has been first applied as an anode material for sodium‐ion batteries (SIBs) with ether‐based electrolytes. The CoSe₂ delivers excellent sodiation and desodiation properties when using 1 m NaCF₃SO₃ in diethyleneglycol dimethylether as an electrolyte and cycling between 0.5 and 3.0 V. A high discharge capacity of 0.410 Ah g^?1 is obtained at 1 A g^?1 after 1800 cycles, corresponding to a capacity retention of 98.6% calculated from the 30th cycle. Even at an ultrahigh rate of 50 A g^?1, the capacity still maintains 0.097 Ah g^?1. The reaction mechanism of the as‐prepared CoSe₂ is also investigated. The results demonstrate that at discharged 1.56 V, insertion reaction occurs, while two conversion reactions take place at the second and third plateaus around 0.98 and 0.65 V. During the charge process, Co first reacts with Na₂Se to form Na_xCoSe₂ and then turns back to CoSe₂. In addition to Na/CoSe₂ half cells, Na₃V₂(PO₄)₃/CoSe₂ full cell with excessive amount of Na₃V₂(PO₄)₃ has been studied. The full cell exhibits a reversible capacity of 0.380 Ah g^?1. This work definitely enriches the possibilities for anode materials for SIBs with high performance.     

FeO0.7F1.3/C Nanocomposite as a High‐Capacity Cathode Material for Sodium‐Ion Batteries

Searching high capacity cathode materials is one of the most important fields of the research and development of sodium‐ion batteries (SIBs). Here, we report a FeO_0.7F_1.3/C nanocomposite synthesized via a solution process as a new cathode material for SIBs. This material exhibits a high initial discharge capacity of 496 mAh g^?1 in a sodium cell at 50 °C. From the 3^rd to 50^th cycle, the capacity fading is only 0.14% per cycle (from 388 mAh g^?1 at 3^rd the cycle to 360 mAh g^?1 at the 50^th cycle), demonstrating superior cyclability. A high energy density of 650 Wh kg^?1 is obtained at the material level. The reaction mechanism studies of FeO_0.7F_1.3/C with sodium show a hybridized mechanism of both intercalation and conversion reaction.     

Layer‐Based Heterostructured Cathodes for Lithium‐Ion and Sodium‐Ion Batteries

A critical bottleneck that hinders major performance improvement in lithium‐ion and sodium‐ion batteries is the inferior electrochemical activity of their cathode materials. While significant research progresses have been made, conventional single‐phase cathodes are still limited by intrinsic deficiencies such as low reversible capacity, enormous initial capacity loss, rapid capacity decay, and poor rate capability. In the past decade, layer‐based heterostructured cathodes acquired by combining multiple crystalline phases have emerged as candidates with a huge potential to realize performance breakthrough. Herein, recent studies on the structural properties, electrochemical behaviors, and synthesis route optimizations of these heterostructured cathodes are summarized for in‐depth discussions. Particular attention is paid to the latest mechanism discoveries and performance achievements. This review thus aims to promote a deeper understanding of the correlation between the crystal structure of cathodes and their electrochemical behavior, and offers guidance to design advance cathode materials from the aspect of crystal structure engineering.     

Flexible Stable Solid‐State Al‐Ion Batteries

Rechargeable aluminum‐ion batteries (AIBs) are regarded as promising candidates for post‐lithium energy storage systems (ESSs). For addressing the critical issues in the current liquid AIB systems, here a flexible solid‐state AIB is established using a gel‐polymer electrolyte for achieving robust electrode–electrolyte interfaces. Different from utilization of solid‐state systems for alleviating the safety issues and enhancing energy density in lithium‐ion batteries, employment of polymeric electrolytes mainly focuses on addressing the essential problems in the liquid AIBs, including unstable internal interfaces induced by mechanical deformation and production of gases as well as unfavorable separators. Particularly, such gel electrolyte enables the solid‐state AIBs to present an ultra‐fast charge capability within 10 s at current density of 600 mA g^?1. Meanwhile, an impressive specific capacity ≈120 mA h g^?1 is obtained at current density of 60 mA g^?1, approaching the theoretical limit of graphite‐based AIBs. In addition to the well‐retained electrochemical performance below the ice point, the solid‐state AIBs also hold great stability and safety under various critical conditions. The results suggest that such new prototype of solid‐state AIBs with robust electrode–electrolyte interfaces promises a novel strategy for fabricating stable and safe flexible ESSs.     

Reversible and High‐Capacity Nanostructured Electrode Materials for Li‐Ion Batteries

Reversible nanostructured electrode materials are at the center of research relating to rechargeable lithium batteries, which require high power, high capacity, and high safety. The higher capacities and higher rate capabilities for the nanostructured electrode materials than for the bulk counterparts can be attributed to the higher surface area, which reduces the overpotential and allows faster reaction kinetics at the electrode surface. These electrochemical enhancements can lead to versatile potential applications of the batteries and can provide breakthroughs for the currently limited power suppliers of mobile electronics. This Feature Article describes recent research advances on nanostructured cathode and anode materials, such as metals, metal oxides, metal phosphides and LiCoO₂, LiNi_1–xM_xO₂ with zero‐, one‐, two‐, and three‐dimensional morphologies.     

Electrode Engineering by Atomic Layer Deposition for Sodium‐Ion Batteries: From Traditional to Advanced Batteries

Sodium‐ion batteries (SIBs) have emerged as one of the most promising and competitive energy storage systems due to abundant sodium resources and its environmentally friendly features. However, further improvements in the engineering of the SIB electrode/electrolyte interphase—which directly determines the Na‐ion transfer behavior, material structure stability, and sodiation/desodiation property—are highly recommended to meet the continuously increasing requirements for secondary power sources. Reasonably speaking, to promote SIBs, the advanced and controllable interphase/electrode engineering approach exhibits promise by rationally designing the bulk electrode and generating a well‐defined interphase. Atomic layer deposition (ALD) technology, with atomic‐scale deposition, superior uniformity, excellent conformality, and a self‐limiting nature, is thus expected to address the current challenges facing SIBs in terms of low energy density, limited cycling life, and structural instability, and to promote innovations such as multifunctional electrodes and nanostructured materials for advanced SIBs. This review summarizes and discusses the most recent advancements in the interphase engineering of SIBs by ALD via modifying traditional electrodes and designing advanced electrodes (such as 3D, organic, and protected sodium metal electrodes). Furthermore, based on the recent critical progress and current scientific understanding, future perspectives for the engineering of next‐generation SIB electrodes by ALD can be provided.     

MXene‐Bonded Flexible Hard Carbon Film as Anode for Stable Na/K‐Ion Storage

Hard carbon (HC) is a promising anode material for sodium‐ion batteries (SIBs) and potassium‐ion batteries (PIBs), but the volume change during the insertion/extraction of Na⁺ or K⁺ limits the cycle life, especially for PIBs due to the large ion size of K⁺. Moreover, the conventional anodes fabricated through the coating method cannot satisfy the requirement of flexible devices. Here, it is shown that 2D carbide flakes of Ti₃C₂T_x MXene can be used as multifunctional conductive binders for flexible HC electrodes. The use of MXene nanosheets eliminates the need for all the electrochemically inactive components in the conventional polyvinylidene fluoride–bonded HC electrode, including polymer binders, conductive additives, and current collectors. In MXene‐bonded HC electrodes, conductive and hydrophilic MXene 2D nanosheets construct a 3D network, which can effectively stabilize the electrode structure and accommodate the volume expansion of HC during the charge/discharge process, leading to an enhanced electrode capacity and excellent cycle performance as anodes for both SIBs and PIBs. Benefiting from the 3D conductive network, the MXene‐bonded HC film electrodes also present improved rate capability, indicating MXene is a very promising multifunctional binder for next‐generation flexible secondary rechargeable batteries.     

Engineering Hierarchical Hollow Nickel Sulfide Spheres for High‐Performance Sodium Storage

Sodium‐ion batteries (SIBs) are considered as promising alternatives to lithium‐ion batteries (LIBs) for energy storage due to the abundance of sodium, especially for grid distribution systems. The practical implementation of SIBs, however, is severely hindered by their low energy density and poor cycling stability due to the poor electrochemical performance of the existing electrodes. Here, to achieve high‐capacity and durable sodium storage with good rate capability, hierarchical hollow NiS spheres with porous shells composed of nanoparticles are designed and synthesized by tuning the reaction parameters. The formation mechanism of this unique structure is systematically investigated, which is clearly revealed to be Ostwald ripening mechanism on the basis of the time‐dependent morphology evolution. The hierarchical hollow structure provides sufficient electrode/electrolyte contact, shortened Na⁺ diffusion pathways, and high strain‐tolerance capability. The hollow NiS spheres deliver high reversible capacity (683.8 mAh g^?1 at 0.1 A g^?1), excellent rate capability (337.4 mAh g^?1 at 5 A g^?1), and good cycling stability (499.9 mAh g^?1 with 73% retention after 50 cycles at 0.1 A g^?1).     

Super Kinetically Pseudocapacitive MnCo2S4 Nanourchins toward High‐Rate and Highly Stable Sodium‐Ion Storage

Improving surface morphology profiles, i.e., surface area and porosity, by nanostructure/surface engineering is effective in accommodating sodium's ionic and kinetic inadequacies. However, this strategy is limited to only activating the extrinsic pseudocapacitance in terms of improving surface‐based reactions. Herein, it is aimed to improve the sodiation performance by enhancement from both intrinsic and extrinsic pseudocapacitance to maximize sodiation potential of materials. A rarely reported but highly functional spinel MnCo₂S₄ (MCS), is introduced and systematically analyzed using first‐principles investigations, which exhibits energetically favorable charge‐transfer states and strong Na‐ions adsorption kinetics as well as diffusion channels (?3.65 and 0.40 eV respectively). The overall electrochemical redox profiles of the MCS nanostructure is revealed by in situ techniques, which disclose the commencing of partial and then a full conversion‐type sodiation at low discharge potentials (0.52 V vs Na/Na⁺) with fast Na‐ions diffusivity. Assisted by surface engineering technology on the intrinsically pseudocapacitive MCS, the urchin‐like morphology is instrumental in boosting and realizing sodium storage performance, especially the surface capacitive behavior (from 73.4% to 94.1%), prolonged cycling stability (>800 cycles), and high‐rate capability (416 mAh g^?1 at 10 A g^?1), as well as exhibiting remarkable full cell capability (high rate at 2 A g^?1, >200 cycles at 200 mA g^?1).     

Three Electron Reversible Redox Reaction in Sodium Vanadium Chromium Phosphate as a High‐Energy‐Density Cathode for Sodium‐Ion Batteries

A sodium‐ion battery operating at room temperature is of great interest for large‐scale stationary energy storage because of its intrinsic cost advantage. However, the development of a high capacity cathode with high energy density remains a great challenge. In this work, sodium super ionic conductor‐structured Na₃V_2?xCr_x(PO₄)₃ is achieved through the sol–gel method; Na₃V_1.5Cr_0.5(PO₄)₃ is demonstrated to have a capacity of 150 mAh g^?1 with reversible three‐electron redox reactions after insertion of a Na⁺, consistent with the redox couples of V²⁺/³⁺, V³⁺/⁴⁺, and V⁴⁺/⁵⁺. Moreover, a symmetric sodium‐ion full cell utilizing Na₃V_1.5Cr_0.5(PO₄)₃ as both the cathode and anode exhibits an excellent rate capability and cyclability with a capacity of 70 mAh g^?1 at 1 A g^?1. Ex situ X‐ray diffraction analysis and in situ impedance measurements are performed to reveal the sodium storage mechanism and the structural evolution during cycling.     

Electronic Structure Regulation of Layered Vanadium Oxide via Interlayer Doping Strategy toward Superior High‐Rate and Low‐Temperature Zinc‐Ion Batteries

Currently, development of suitable cathode materials for zinc‐ion batteries (ZIBs) is plagued by the sluggish kinetics of Zn²⁺ with multivalent charge in the host structure. Herein, it is demonstrated that interlayer Mn²⁺‐doped layered vanadium oxide (Mn_0.15V₂O₅·nH₂O) composites with narrowed direct bandgap manifest greatly boosted electrochemical performance as zinc‐ion battery cathodes. Specifically, the Mn_0.15V₂O₅·nH₂O electrode shows a high specific capacity of 367 mAh g^?1 at a current density of 0.1 A g^?1 as well as excellent retentive capacities of 153 and 122 mAh g^?1 after 8000 cycles at high current densities up to 10 and 20 A g^?1, respectively. Even at a low temperature of ?20 °C, a reversible specific capacity of 100 mAh g^?1 can be achieved at a current density of 2.0 A g^?1 after 3000 cycles. The superior electrochemical performance originates from the synergistic effects between the layered nanostructures and interlayer doping of Mn²⁺ ions and water molecules, which can enhance the electrons/ions transport kinetics and structural stability during cycling. With the aid of various ex situ characterization technologies and density functional theory calculations, the zinc‐ion storage mechanism can be revealed, which provides fundamental guidelines for developing high‐performance cathodes for ZIBs.     

One‐Pot Synthesis of Antimony‐Embedded Silicon Oxycarbide Materials for High‐Performance Sodium‐Ion Batteries

Sodium‐ion batteries have recently attracted intensive attention due to their natural abundance and low cost. Antimony is a desirable candidate for an anode material for sodium‐ion batteries due to its high theoretical capacity (660 mA h g^?1). However, the utilization of alloy‐based anodes is still limited by their inherent huge volume changes and sluggish kinetics. The Sb‐embedded silicon oxycarbide (SiOC) composites are simply synthesized via a one‐pot pyrolysis process at 900 °C without any additives or surfactants, taking advantage of the superior self‐dispersion properties of antimony acetate powders in silicone oil. The structural and morphological characterizations confirm that Sb nanoparticles are homogeneously embedded into the amorphous SiOC matrix. The composite materials exhibit an initial desodiation capacity of around 510 mA h g^?1 and maintained an excellent capacity retention above 97% after 250 cycles. The rate capability test reveals that the composites deliver capacity greater than 453 mA h g^?1, even at the high current density of 20 C rate, owing to the free‐carbon domain of SiOC material. The electrochemical and postmortem analyses confirm that the SiOC matrix with a uniform distribution of Sb nanoparticles provides the mechanical strength without degradation in conductive characteristics, suppressing the agglomeration of Sb particles during the electrochemical reaction.     

Graphene Oxide‐Template Controlled Cuboid‐Shaped High‐Capacity VS4 Nanoparticles as Anode for Sodium‐Ion Batteries

Room‐temperature sodium‐ion batteries have attracted great attentions for large‐scale energy storage applications in renewable energy. However, exploring suitable anode materials with high reversible capacity and cyclic stability is still a challenge. The VS₄, with parallel quasi‐1D chains structure of V⁴⁺(S₂^2?)₂, which provides large interchain distance of 5.83 Å and high capacity, has showed great potential for sodium storage. Here, the uniform cuboid‐shaped VS₄ nanoparticles are prepared as anode for sodium‐ion batteries by the controllable of graphene oxide (GO)‐template contents. It exhibits superb electrochemical performances of high‐specific charge capacity (≈580 mAh·g^?1 at 0.1 A·g^?1), long‐cycle‐life (≈98% retain at 0.5 A·g?¹ after 300 cycles), and high rates (up to 20 A·g^?1). In addition, electrolytes are optimized to understand the sodium storage mechanism. It is thus demonstrated that the findings have great potentials for the applications in high‐performance sodium‐ion batteries.