The Application of Hollow Structured Anodes for Sodium-Ion Batteries: From Simple to Complex Systems

Tannic acid etching construction of hollow heterogeneous CoSe2-FeSe2@nitrogen-doped carbon rhombic dodecahedron for high-performance sodium storage

Metal selenides are very promising anode materials for sodium ion batteries (SIBs) due to their rich redox behaviors, low cost, high theoretical capacity, and environmentally benign. However, the poor cycle performance and rate capability greatly hinder their widespread applications. In this paper, we have proposed a tannic acid etching zeolitic imidazolate framework-67 (ZIF-67)-derived selenide strategy to construct hollow heterogeneous CoSe2-FeSe2@N-doped carbon rhombic dodecahedron (CoSe2-FeSe2@NC) as anode for high-performance SIBs. The special microstructural characteristics with hollow rhombic dodecahedron can reduce the Na+/electron migration path and alleviate the volume variations during cycling. The NC can improve conductivity and reduce volume effects during cycling. What's more, the built-in electric fields (BIEF) at the CoSe2-FeSe2 heterointerfaces can modulate the electronic structure and accelerate the kinetics of ionic diffusion, resulting in the improvement electrochemical properties. When applied as anodes for SIBs, the CoSe2-FeSe2@NC can deliver a remarkable electrochemical performance in terms of sodium storage capacity (648.5 mAh g-1 at 0.2 A/g), initial coulombic efficiency (82.0 %), cycle performance (92.6 % capacity retention after 100 cycles), and rate capability of 450.6 mAh g-1 after 1000 cycles at a high rate of 1 A/g. The kinetic analysis indicates that the discharging-charging process of CoSe2-FeSe2@NC is ascribed to both capacitive behavior and controlled diffusion.

Novel nitrogen-doped carbon-coated SnSe2 based on a post-synthetically modified MOF as a high-performance anode material for LIBs and SIBs

SnSe2 with high theoretical capacity has been identified as an emerging anode candidate for lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs). However, the rate performance and cycling performance of this material in practical applications are still limited by unavoidable volume expansion and low conductivity. In this work, we designed and synthesized nitrogen-doped carbon-coated SnSe2/C-N composites using 2-aminoterephthalic acid (C8H7NO4) as a nitrogen-containing compound for modification by hydrothermal and vacuum calcination methods to achieve efficient utilization of active sites and optimization of the electronic structure. The carbon skeleton inherited from the Sn-MOF precursor can effectively improve the electronic conduction properties of SnSe2. N-doping in the Sn-MOF can increase the positive and negative electrostatic potential energy regions on the molecular surface to further improve the electrical conductivity, and effectively reduce the binding energy with Li+/Na+ which was determined by Density Functional Theory (DFT) methods. In addition, the N-doped carbon skeleton also introduces a larger space for Li+/Na+ intercalation and enhances the mechanical properties. In particular, the post-synthetically modified MOF-derived SnSe2/C-N materials exhibit excellent cyclability, with a reversible capacity of 695 mA h g-1 for LIBs and 259 mA h g-1 for SIBs after 100 cycles at 100 mA g-1.

Preferred crystal plane electrodeposition of aluminum anode with high lattice-matching for long-life aluminum batteries

Aluminum batteries have become the most attractive next-generation energy storage battery due to their advantages of high safety, high abundance, and low cost. However, the dendrite problem associated with inhomogeneous electrodeposition during cycling leads to low Coulombic efficiency and rapid short-circuit failure of the aluminum metal anode, which severely hampers the cycling stability of aluminum battery. Here we show an aluminum anode material that achieves high lattice matching between the substrate and the deposit, allowing the aluminum deposits to maintain preferred crystal plane growth on the substrate surface. It not only reduces the nucleation barrier of aluminum and decreases electrode polarization, but also enables uniform deposition of aluminum, improving the cycling stability of aluminum batteries. Aluminum anode with (111) preferred crystal plane can stably 25000 cycles at the current density of 5 A·g-1, with a capacity retention rate of over 80%.

Rod-like Ni-CoS2@NC@C: Structural design, heteroatom doping and carbon confinement engineering to synergistically boost sodium storage performance

Currently, conversion-type transition metal sulfides have been extensively favored as the anodes for sodium-ion batteries due to their excellent redox reversibility and high theoretical capacity; however, they generally suffer from large volume expansion and structural instability during repeatedly Na+ de/intercalation. Herein, spatially dual-confined Ni-doped CoS2@NC@C microrods (Ni-CoS2@NC@C) are developed via structural design, heteroatom doping and carbon confinement to boost sodium storage performance of the material. The morphology of one-dimensional-structured microrods effectively enlarges the electrode/electrolyte contact area, while the confinement of dual-carbon layers greatly alleviates the volume change-induced stress, pulverization, agglomeration of the material during charging and discharging. Moreover, the introduction of Ni improves the electrical conductivity of the material by modulating the electronic structure and enlarges the interlayer distance to accelerate Na+ diffusion. Accordingly, the as-prepared Ni-CoS2@NC@C exhibits superb electrochemical properties, delivering the satisfactory cycling performance of 526.6 mA h g-1 after 250 cycles at 1 A g-1, excellent rate performance of 410.9 mA h g-1 at 5 A g-1 and superior long cycling life of 502.5 mA h g-1 after 1,500 cycles at 5 A g-1. This study provides an innovative idea to improve sodium storage performance of conversion-type transition metal sulfides through the comprehensive strategy of structural design, heteroatom doping and carbon confinement.

Metal Phosphide Anodes in Sodium-Ion Batteries: Latest Applications and Progress

Sodium-ion batteries (SIBs), as the next-generation high-performance electrochemical energy storage devices, have attracted widespread attention due to their cost-effectiveness and wide geographical distribution of sodium. As a crucial component of the structure of SIBs, the anode material plays a crucial role in determining its electrochemical performance. Significantly, metal phosphide exhibits remarkable application prospects as an anode material for SIBs because of its low redox potential and high theoretical capacity. However, due to volume expansion limitations and other factors, the rate and cycling performance of metal phosphides have gradually declined. To address these challenges, various viable solutions have been explored. In this paper, the recent research progress of metal phosphide materials for SIBs is systematically reviewed, including the synthesis strategy of metal phosphide, the storage mechanism of sodium ions, and the application of metal phosphide in electrochemical aspects. In addition, future challenges and opportunities based on current developments are presented.

Room-Temperature Salt Template Synthesis of Nitrogen-Doped 3D Porous Carbon for Fast Metal-Ion Storage

The water-soluble salt-template technique holds great promise for fabricating 3D porous materials. However, an equipment-free and pore-size controllable synthetic approach employing salt-template precursors at room temperature has remained unexplored. Herein, we introduce a green room-temperature antisolvent precipitation strategy for creating salt-template self-assembly precursors to universally produce 3D porous materials with controllable pore size. Through a combination of theoretical simulations and advanced characterization techniques, we unveil the antisolvent precipitation mechanism and provide guidelines for selecting raw materials and controlling the size of precipitated salt. Following the calcination and washing steps, we achieve large-scale and universal production of 3D porous materials and the recycling of the salt templates and antisolvents. The optimized nitrogen-doped 3D porous carbon (N-3DPC) materials demonstrate distinctive structural benefits, facilitating a high capacity for potassium-ion storage along with exceptional reversibility. This is further supported by in situ electrochemical impedance spectra, in situ Raman spectroscopy, and theoretical calculations. The anode shows a high rate capacity of 181 mAh g-1 at 4 A g-1 in the full cell. This study addresses the knowledge gap concerning the room-temperature synthesis of salt-template self-assembly precursors for the large-scale production of porous materials, thereby expanding their potential applications for electrochemical energy conversion and storage.

Rational Design of Hollow Structural Materials for Sodium-Ion Battery Anodes

The development of sodium-ion battery (SIB) anodes is still hindered by their rapid capacity decay and poor rate capabilities. Although there have been some new materials that can be used to fabricate stable anodes, SIBs are still far from wide applications. Strategies like nanostructure construction and material modification have been used to prepare more robust SIB anodes. Among all the design strategies, the hollow structure design is a promising method in the development of advanced anode materials. In the past decade, research efforts have been devoted to modifying the synthetic route, the type of templates, and the interior structure of hollow structures with high capacity and stability. A brief introduction is made to the main material systems and classifications of hollow structural materials first. Then different morphologies of hollow structural materials for SIB anodes from the latest reports are discussed, including nanoboxes, nanospheres, yolk shells, nanotubes, and other more complex shapes. The most used templates for the synthesis of hollow structrual materials are covered and the perspectives are highlighted at the end. This review offers a comprehensive discussion of the synthesis of hollow structural materials for SIB anodes, which could be potentially of use to research areas involving hollow materials design for batteries.

Homologous Heterostructured NiS/NiS2 @C Hollow Ultrathin Microspheres with Interfacial Electron Redistribution for High-Performance Sodium Storage

Nickel sulfides with high theoretical capacity are considered as promising anode materials for sodium-ion batteries (SIBs); however, their intrinsic poor electric conductivity, large volume change during charging/discharging, and easy sulfur dissolution result in inferior electrochemical performance for sodium storage. Herein, a hierarchical hollow microsphere is assembled from heterostructured NiS/NiS2 nanoparticles confined by in situ carbon layer (H-NiS/NiS2 @C) via regulating the sulfidation temperature of the precursor Ni-MOFs. The morphology of ultrathin hollow spherical shells and confinement of in situ carbon layer to active materials provide rich channels for ion/electron transfer and alleviate the effects of volume change and agglomeration of the material. Consequently, the as-prepared H-NiS/NiS2 @C exhibit superb electrochemical properties, satisfactory initial specific capacity of 953.0 mA h g-1 at 0.1 A g-1 , excellent rate capability of 509.9 mA h g-1 at 2 A g-1 , and superior longtime cycling life with 433.4 mA h g-1 after 4500 cycles at 10 A g-1 . Density functional theory calculation shows that heterogenous interfaces with electron redistribution lead to charge transfer from NiS to NiS2 , and thus favor interfacial electron transport and reduce ion-diffusion barrier. This work provides an innovative idea for the synthesis of homologous heterostructures for high-efficiency SIB electrode materials.

Hybridization of Layered Titanium Oxides and Covalent Organic Nanosheets into Hollow Spheres for High-Performance Sodium-Ion Batteries with Boosted Electrical/Ionic Conductivity and Ultralong Cycle Life

While development of a sodium-ion battery (SIB) cathode has been approached by various routes, research on compatible anodes for advanced SIB systems has not been sufficiently addressed. The anode materials based on titanium oxide typically show low electrical performances in SIB systems primarily due to their low electrical/ionic conductivity. Thus, in this work, layered titanium oxides were hybridized with covalent organic nanosheets (CONs), which exhibited excellent electrical conductivity, to be used as anodes in SIBs. Moreover, to enlarge the accessible areas for sodium ions, the morphology of the hybrid was formulated in the form of a hollow sphere (HS), leading to the highly enhanced ionic conductivity. This synthesis method was based on the expectation of synergetic effects since titanium oxide provides direct electrostatic sodiation sites that shield organic components and CON supports high electrical and ionic conductivity with polarizable sodiation sites. Therefore, the hybrid shows enhanced and stable electrochemical performances as an anode for up to 2600 charge/discharge cycles compared to the HS without CONs. Furthermore, the best reversible capacities obtained from the hybrid were 426.2 and 108.5 mAh/g at current densities of 100 and 6000 mA/g, which are noteworthy results for the TiO₂-based material.

Vanadium oxide nanorods as an electrode material for solid state supercapacitor

The electrochemical properties of metal oxides are very attractive and fascinating in general, making them a potential candidate for supercapacitor application. Vanadium oxide is of particular interest because it possesses a variety of valence states and is also cost effective with low toxicity and a wide voltage window. In the present study, vanadium oxide nanorods were synthesized using a modified sol-gel technique at low temperature. Surface morphology and crystallinity studies were carried out by using scanning electron microscopy, transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy analysis. To the best of our knowledge, the as-prepared nanorods were tested with magnesium ion based polymer gel electrolyte for the first time. The prepared supercapacitor cell exhibits high capacitance values of the order of ~ 141.8 F g^-1 with power density of ~ 2.3 kW kg^-1 and energy density of ~ 19.1 Wh kg^-1. The cells show excellent rate capability and good cycling stability.

Enhanced High-Rate Capability and Long Cycle Stability of FeS@NCG Nanofibers for Sodium-Ion Battery Anodes

The development of advanced hierarchical anode materials has recently become essential to achieving high-performance sodium-ion batteries. Herein, we developed a facile and cost-effective scheme for synthesizing graphene-wrapped, nitrogen-rich carbon-coated iron sulfide nanofibers (FeS@NCG) as an anode for SIBs. The designed FeS@NCG can provide a significant reversible capacity of 748.5 mAh g^-1 at 0.3 A g^-1 for 50 cycles and approximately 3.9-fold higher electrochemical performance than its oxide analog (Fe₂O₃@NCG, 192.7 mAh g^-1 at 0.3 A g^-1 for 50 cycles). The sulfur- and nitrogen-rich multilayer package structure facilitates efficient suppression of the porous FeS volume expansion during the sodiation process, enabling a long cycle life. The intimate contact between graphene and porous carbon-coated FeS nanofibers offers strong structural barriers associated with charge-transfer pathways during sodium insertion/extraction. It also reduces the dissolution of polysulfides, enabling efficient sodium storage with superior stable kinetics. Furthermore, outstanding capacity retention of 535 mAh g^-1 at 5 A g^-1 is achieved over 1010 cycles. The FeS@NCG also exhibited a specific capacity of 640 mAh g^-1 with a Coulombic efficiency of above 99.8% at 5 A g^-1 at 80 °C, indicating its development prospects in high-performance SIB applications.

Mechanisms for self-templating design of micro/nanostructures toward efficient energy storage

The ever-growing demand in modern power systems calls for the innovation in electrochemical energy storage devices so as to achieve both supercapacitor-like high power density and battery-like high energy density. Rational design of the micro/nanostructures of energy storage materials offers a pathway to finely tailor their electrochemical properties thereby enabling significant improvements in device performances and enormous strategies have been developed for synthesizing hierarchically structured active materials. Among all strategies, the direct conversion of precursor templates into target micro/nanostructures through physical and/or chemical processes is facile, controllable, and scalable. Yet the mechanistic understanding of the self-templating method is lacking and the synthetic versatility for constructing complex architectures is inadequately demonstrated. This review starts with the introduction of five main self-templating synthetic mechanisms and the corresponding constructed hierarchical micro/nanostructures. Subsequently, the structural merits provided by the well-defined architectures for energy storage are elaborately discussed. At last, a summary of current challenges and future development of the self-templating method for synthesizing high-performance electrode materials is also presented.

Bean Pod-Like SbSn/N-Doped Carbon Fibers toward a Binder Free, Free-Standing, and High-Performance Anode for Sodium-Ion Batteries

Bimetallic SbSn alloy stands out among the anode materials for sodium-ion batteries (SIBs) because of its high theoretical specific capacity (752 mAh g^-1 ) and good electrical conductivity. However, the major challenge is the large volume change during cycling processes, bringing about rapid capacity decay. Herein, to cope with this issue, through electrostatic spinning and high temperature calcination reduction, the unique bean pod-like free-standing membrane is designed initially, filling SbSn dots into integrated carbon matrix including hollow carbon spheres and nitrogen-doped carbon fibers (B-SbSn/NCFs). Significantly, the synergistic carbon matrix not only improves the conductivity and flexibility, but provides enough buffer space to alleviate the large volume change of metal particles. More importantly, the B-SbSn/NCFs free-standing membrane can be directly used as the anode without polymer binder and conductive agent, which improves the energy density and reaction kinetics. Satisfyingly, the free-standing BSbSn/NCFs membrane anode shows excellent electrochemical performance in SIB. The specific capacity of the membrane electrode can maintain 486.9 mAh g^-1 and the coulombic efficiency is close to 100% after 400 cycles at 100 mA g^-1 . Furthermore, the full cell based on B-SbSn/NCFs anode also exhibits the good electrochemical performance.

Sodium-Ion Battery with a Wide Operation-Temperature Range from -70 to 100 °C

Sodium-ion batteries (SIBs), as one of the potential candidates for grid-scale energy storage systems, are required to tackle extreme weather conditions. However, the all-weather SIBs with a wide operation-temperature range are rarely reported. Herein, we propose a wide-temperature range SIB, which involves a carbon-coated Na₄ Fe₃ (PO₄ )₂ P₂ O₇ (NFPP@C) cathode, a bismuth (Bi) anode, and a diglyme-based electrolyte. We demonstrate that solvated Na⁺ can be directly stored by the Bi anode via an alloying reaction without the de-solvent process. Furthermore, the NFPP@C cathode exhibits a high Na⁺ diffusion coefficient at low temperature. As a result, the Bi//NFPP@C battery exhibits perfect low-temperature behavior. Even at -70 °C, this battery still delivers 70.19 % of the room-temperature capacity. Furthermore, benefitting from the high boiling point of the electrolyte, this battery also works well at a high temperature of up to 100 °C. These results are encouraging for the further exploration of wide-temperature range SIBs.

N, P Codoped Hollow Carbon Nanospheres Decorated with MoSe2 Ultrathin Nanosheets for Efficient Potassium-Ion Storage

Potassium-ion batteries (KIBs) are gradually being considered as an alternative for lithium-ion batteries because of their non-negligible advantages such as abundance and low expenditure of K, as well as higher electrochemical potential than another alternative─sodium-ion batteries. Nevertheless, when the electrode materials are inserted and extracted with large-sized K⁺ ions, the tremendous volume change will cause the collapse of the microstructures of electrodes and make the charging/discharging process irreversible, thus disapproving their extended application. In response to this, we put forward a feasible strategy to realize the in situ assembly of layered MoSe₂ nanosheets onto N, P codoped hollow carbon nanospheres (MoSe₂/NP-HCNSs) through thermal annealing and heteroatom doping strategies, and the resulting nanoengineered material can function well as an anode for KIBs. This cleverly designed nanostructure of MoSe₂/NP-HCNS can broaden the interlayer spacing of MoSe₂ to boost the efficiency of the insertion/extraction of K ions and also can accommodate large volume change-caused mechanical strain, facilitate electrolyte penetration, and prevent the aggregation of MoSe₂ nanosheets. This synthetic method generates abundant defects to increase the amounts of active sites, as well as conductivity. The hierarchical nanostructure can effectively increase the proportion of pseudo-capacitance and promote interfacial electronic transfer and K⁺ diffusion, thus imparting great electrochemical performance. The MoSe₂/NP-HCNS anode exhibits a high reversible capacity of 239.9 mA h g^-1 at 0.1 A g^-1 after 200 cycles and an ultralong cycling life of 161.1 mA h g^-1 at 1 A g^-1 for a long period of 1000 cycles. This nanoengineering method opens up new insights into the development of promising anode materials for KIBs.

Environment-friendly degradable zinc-ion battery based on guar gum-cellulose aerogel electrolyte

With the vigorous development of electronics and the increasingly prominent problem of environmental pollution, it is particularly important to exploit environmentally friendly electronic devices. Transient electronics represent a kind of device that once the specified functions have completed can completely or partially disappear through physical or chemical actions. In this work, we introduce a novel guar gum-cellulose aerogel (GCA) membrane based on natural biomaterials and successfully use it as an electrolyte film to fabricate a degradable zinc-ion battery (DZIB). All components of the prepared DZIBs can be successfully degraded or disintegrate in phosphate-buffered saline (PBS) containing a solution of proteinase K after approximately 40 days. This electrolyte film has a high ionic conductivity of approximately 4.73 × 10^-2 S cm^-1 and a good mechanical stress property. When applied to DZIB, the production of zinc dendrites can be restrained, leading to the battery showing excellent electrochemical performance. The battery exhibits a specific capacity of 309.1 mA h g^-1 at a current density of 308 mA g^-1 after 100 cycles and a steady cycling ability (100% capacity retention after 200 cycles). More importantly, the electrochemical performance of DZIB is better than that of transient batteries reported in the past, taking a solid step in the field of transient electronics in the initial stage.

Cage-Confinement Pyrolysis Strategy to Synthesize Hollow Carbon Nanocage-Coated Copper Phosphide for Stable and High-Capacity Potassium-Ion Storage

Metal phosphides with a high theoretical capacity and low redox potential have been proposed as promising anodes for potassium-ion batteries (PIBs). A reasonable configuration design and introduction of a hollow structure with adequate internal void spaces are effective strategies to overcome the volume expansion of metal phosphides in potassium-ion batteries. Herein, we report a cage-confinement pyrolysis strategy to obtain hollow nanocage-structured nitrogen/phosphorus dual-doped carbon-coated copper phosphide (Cu3P/CuP2@NPC), which exhibits a high initial charge capacity (409 mA h g-1 at 100 mA g-1) and an outstanding cycle performance (100 mA h g-1 after 5000 cycles at 1000 mA g-1) as an anode material for PIBs. The novel hollow nanocage structure could prevent volume expansion during cycling and reduce the electron/ion diffusion distance. Besides, the nitrogen/phosphorus dual-doped carbon-coated layer could promote electronic conductivity. In situ X-ray diffraction (XRD) measurements are conducted to study the potassiation/depotassiation mechanism of Cu3P/CuP2@NPC and reveal the structure stability during the cycle process, which further proves that the design ideas of the conductive carbon layer and the hollow structure with adequate internal void spaces are successful.

Spiral self-assembly of lamellar micelles into multi-shelled hollow nanospheres with unique chiral architecture

Functional carbon nanospheres are exceptionally useful, yet controllable synthesis of them with well-defined porosity and complex multi-shelled nanostructure remains challenging. Here, we report a lamellar micelle spiral self-assembly strategy to synthesize multi-shelled mesoporous carbon nanospheres with unique chirality. This synthesis features the introduction of shearing flow to drive the spiral self-assembly, which is different from conventional chiral templating methods. Furthermore, a continuous adjustment in the amphipathicity of surfactants can cause the packing parameter changes, namely, micellar structure transformations, resulting in diverse pore structures from single-porous, to radial orientated, to flower-like, and to multi-shelled configurations. The self-supported spiral architecture of these multi-shelled carbon nanospheres, in combination with their high surface area (~530 m² g⁻¹), abundant nitrogen content (~6.2 weight %), and plentiful mesopores (~2.5 nm), affords them excellent electrochemical performance for potassium-ion storage. This simple but powerful micelle-directed self-assembly strategy offers inspiration for future nanostructure design of functional materials.

Hollow MoS2/Co nanopillars with boosted Li-ion diffusion rate and long-term cycling stability

Hollow MoS₂/Co-0.1 nanopillars were successfully synthesized by sulfurizing CoMoO₄ and subsequent acid etching, which were used as the anode material for lithium ion batteries. The introduction of suitable metal Co into MoS₂ nanopillars effectively accelerates electron/ion transport kinetics, leading to high specific capacity and superior rate capability and cycling stability.

Increasing Accessible Subsurface to Improving Rate Capability and Cycling Stability of Sodium-Ion Batteries

Numerous studies have reported that the enhancement of rate capability of carbonaceous anode by heteroatom doping is due to the increased diffusion-controlled capacity induced by expanding interlayer spacing. However, percentage of diffusion-controlled capacity is less than 30% as scan rate is larger than 1 mV s^-1 , suggesting there is inaccuracy in recognizing principle of improving rate capability of carbonaceous anode. In this paper, it is found that the heteroatom doping has little impact on interlayer spacing of carbon in bulk phase, meaning that diffusion-controlled capacity is hard to be enhanced by doping. After synergizing with tensile stress, however, the interlayer spacing in subsurface region is obviously expanded to 0.40 nm, which will increase the thickness of accessible subsurface region at high current density. So SRNDC-700 electrodes display a high specific capacity of 160.6 and 69.5 mAh g^-1 at 20 and 50 A g^-1 , respectively. Additionally, the high reversibility of carbon structure insures ultralong cycling stability and hence attenuation of SRNDC-700 is only 0.0025% per cycle even at 10 A g^-1 for 6000 cycles. This report sheds new insight into mechanism of improving electrochemical performance of carbonaceous anode by doping and provides a novel design concept for doping carbon.

A "Biconcave-Alleviated" Strategy to Construct Aspergillus niger-Derived Carbon/MoS2 for Ultrastable Sodium Ion Storage

Two-dimensional layered materials commonly face hindered electron transfer and poor structure stability, thus limiting their application in high-rate and long-term sodium ion batteries. In the current study, we adopt finite element simulation to guide the rational design of nanostructures. By calculating the von Mises stress distribution of a series of carbon materials, we find that the hollow biconcave structure could effectively alleviate the stress concentration resulting from expansion. Accordingly, we propose a biconcave-alleviated strategy based on the Aspergillus niger-derived carbon (ANDC) to construct ANDC/MoS₂ with a hollow biconcave structure. The ANDC/MoS₂ is endowed with an excellent long-term cyclability as an anode of sodium ion batteries, delivering a discharge capacity of 496 mAh g^-1 after 1000 cycles at 1 A g^-1. A capacity retention rate of 94.5% is achieved, an increase of almost seven times compared with the bare MoS₂ nanosheets. Even at a high current density of 5 A g^-1, a reversible discharge capacity around 400 mAh g^-1 is maintained after 300 cycles. ANDC/MoS₂ could also be used for efficient lithium storage. By using in situ TEM, we further reveal that the hollow biconcave structure of ANDC/MoS₂ has enabled stable and fast sodiation/desodiation.

A Gas-Steamed MOF Route to P-Doped Open Carbon Cages with Enhanced Zn-Ion Energy Storage Capability and Ultrastability

Carbon micro/nanocages have received great attention, especially in electrochemical energy-storage systems. Herein, as a proof-of-concept, a solid-state gas-steamed metal-organic-framework approach is designed to fabricate carbon cages with controlled openings on walls, and N, P dopants. Taking advantage of the fabricated carbon cages with large openings on their walls for enhanced kinetics of mass transport and N, P dopants within the carbon matrix for favoring chemical adsorption of Zn ions, when used as carbon cathodes for advanced aqueous Zn-ion hybrid supercapacitors (ZHSCs), such open carbon cages (OCCs) display a wide operation voltage of 2.0 V and an enhanced capacity of 225 mAh g^-1 at 0.1 A g^-1 . Also, they exhibit an ultralong cycling lifespan of up to 300 000 cycles with 96.5% capacity retention. Particularly, such OCCs as electrode materials lead to a soft-pack ZHSC device, delivering a high energy density of 97 Wh kg^-1 and a superb power density of 6.5 kW kg^-1 . Further, the device can operate in a wide temperature range from -25 to + 40 °C, covering the temperatures for practical applications in daily life.

Construction of MoS2/Mxene heterostructure on stress-modulated kapok fiber for high-rate sodium-ion batteries

Molybdenum disulfide (MoS₂) has possession of a layered structure and high theoretical capacity, which is a candidate anode material for sodium ion batteries. However, unmodified MoS₂ are inflicted with a poor cycling stability and an inferior rate capability upon charge/discharge processes. Considering that the shape and size of anode materials play a key role in the performance of anode materials, this paper proposes a multi-level composite structure formed by the micro-nano materials based on self-assembled molybdenum disulfide (MoS₂) nanoflowers, Mxene and hollow carbonized kapok fiber (CKF). The micro-nano materials can be connected to form heterojunction and agglomeration can be avoided. The load bearing of heterostructure and stress release of CKF are coordinated to form a double protection mechanism, which improves the conductivity and structural stability of hybrid materials. Based on the above advantages, it has higher specific capacity than pure MoS₂, and has better rate performance (639.3, 409.5, 386.2, 372, 338, 422.8 and 434.7 mAh g^-1 at the current density of 0.05, 0.1, 0.2, 0.5, 1 ,0.1 and 0.05 A·g^-1, respectively). The stress-modulated strategies can provide new insights for the design and construction of transition metal sulfides heterostructures to achieve high performance sodium ion batteries.

Yolk-shell structured CoSe2/C nanospheres as multifunctional anode materials for both full/half sodium-ion and full/half potassium-ion batteries

Transition metal selenides (TMSs) are suitable for SIBs and PIBs owing to their satisfactory theoretical capacity and superior electrical conductivity. However, the large radius of Na+/K+ easily leads to sluggish kinetics and poor conductivity, which hinder the development of SIBs and PIBs. Structure design is an effective method to solve these obstacles. In this study, Co2+ ions combined with glycerol molecules to form self-assembled nanospheres at first, and then they were in situ converted into CoSe2 nanoparticles embedded in a carbon matrix during the selenization process. This structure has three-dimensional ion diffusion channels that can effectively hamper the aggregation of metal compound nanoparticles. Meanwhile, the CoSe2/C of the yolk-shell structure and a large number of pores help alleviate volume expansion and facilitate electrolyte wettability. These structural advantages of CoSe2/C endow it with remarkable electrochemical performances for full/half SIBs and full/half PIBs. The obtained CoSe2/C exhibits superior stability and excellent performance (312.1 mA h g-1 at 4 A g-1 after 1600 cycles) for SIBs. When it is used as an anode material for PIBs, 369.2 mA h g-1 can be retained after 200 cycles at 50 mA g-1 and 248.1 mA h g-1 can be retained after 200 cycles at 500 mA g-1; in addition, CoSe2/C also shows superior rate capacity (186.4 mA h g-1 at 1000 mA g-1). A series of ex situ XRD measurements were adapted to explore the possible conversion mechanism of CoSe2/C as the anode for PIBs. It is worth noting that the full-cell of CoSe2/C//Na3V2(PO4)3@rGO for SIBs and the full-cell of CoSe2/C//PTCDA-450 for PIBs were successfully assembled. The relationship between the structure and performance of CoSe2/C was investigated through density functional theory (DFT).

Rational Design and Engineering of One-Dimensional Hollow Nanostructures for Efficient Electrochemical Energy Storage

The unique structural characteristics of one-dimensional (1D) hollow nanostructures result in intriguing physicochemical properties and wide applications, especially for electrochemical energy storage applications. In this Minireview, we give an overview of recent developments in the rational design and engineering of various kinds of 1D hollow nanostructures with well-designed architectures, structural/compositional complexity, controllable morphologies, and enhanced electrochemical properties for different kinds of electrochemical energy storage applications (i.e. lithium-ion batteries, sodium-ion batteries, lithium-sulfur batteries, lithium-selenium sulfur batteries, lithium metal anodes, metal-air batteries, supercapacitors). We conclude with prospects on some critical challenges and possible future research directions in this field. It is anticipated that further innovative studies on the structural and compositional design of functional 1D nanostructured electrodes for energy storage applications will be stimulated.

Unlocking Rapid and Robust Sodium Storage Performance of Zinc-Based Sulfide via Indium Incorporation

Zinc sulfide (ZnS) exhibits promise in sodium-ion batteries (SIBs) because of its low operation voltage and high theoretical specific capacity. However, pristine ZnS is not adequate in realizing rapid and robust sodium storage owing to its low reversibility, poor structure stability, and sluggish kinetics. To date, most efforts focus on utilizing carbonaceous incorporation to improve its electrochemical performances. Nevertheless, it remains an arduous challenge for realizing superior rate capability while obtaining stable cycling. Herein, inspired by the crystal structure of hexagonal ZnIn₂S₄, which possesses an intrinsic layered feature with larger unit-cell volume versus that of ZnS, indium incorporation is thus deployed as an immediate remedy. In/ex situ investigations combined with density functional theory calculations are conducted to reveal the superior kinetics, high reversibility, and good structure stability of ZnIn₂S₄. Notably, the formed indium-based derivatives during cycling manifest a Na⁺ (de)intercalation process, thereby exciting a synergetic mechanism to stabilize electrochemical cycling. As a result, the electrochemical performances of Zn-based sulfide are significantly improved via the indium incorporation. Furthermore, a full cell based on the ZnIn₂S₄ anode with the superior electrochemical performance is developed. This work provides an effective tactic of heteroatom incorporation for optimizing structure as well as exciting a complementary reaction process toward developing superior anodes for high-performance alkali-ion batteries.

Achieving High Pseudocapacitance Anode by An In Situ Nanocrystallization Strategy for Ultrastable Sodium-Ion Batteries

Conversion/alloying type anodes have shown great promise for sodium-ion batteries (SIBs) because of their high theoretical capacity. However, the poor structural stability derived from the large volume expansion and short lifetime impedes their further practical applications. Herein, we report a novel anode with a pomegranate-like nanostructure of SnP₂O₇ particles homogeneously dispersed in the robust N-doped carbon matrix. For the first time, we make use of in situ self-nanocrystallization to generate ultrafine SnP₂O₇ particles with a short pathway of ions and electrons to promote the reaction kinetics. Ex situ transmission electron microscope (TEM) shows that the average particle size of SnP₂O₇ decreases from 66 to 20 nm successfully based on this unique nanoscale-engineering method. Therefore, the nanoparticles together with the N-doped carbon contribute a high pseudocapacitance contribution. Moreover, the N-doped carbon matrix forms strong interaction with the self-nanocrystallization ultrafine SnP₂O₇ particles, leading to a stable nanostructure without any particle aggregation under a long-cycle operation. Benefiting from these synergistic merits, the SnP₂O₇@C anode shows a high specific capacity of 403 mAh g^-1 at 200 mA g^-1 and excellent cycling stability (185 mAh g^-1 after 4000 cycles at 1000 mA g^-1). This work presents a new route for the effective fabrication of advanced conversion/alloying anodes materials for SIBs.

Surface Polymerization and Controlled Pyrolysis: Tailorable Synthesis of Bumpy Hollow Carbon Spheres for Energy Storage

Architectural design of hollow carbon spheres (HCSs) plays a vital role in improving their performance and expanding applications. The tailorable synthesis of bumpy or asymmetric HCSs with a refined structure remains a challenge. Herein, bumpy HCSs (BHCSs) and bumpy concave HCSs (BCHCSs) have been engineered. The synthesis involves the formation of a core/shell precursor via the surface polymerization of pyrrole monomers on polystyrene nanoparticles, followed by the controlled pyrolysis process under different conditions. In comparison with HCSs, the concave hollow structure can reduce the excessive interior cavity and maintain prevalent merits of hollow structures; the bumpy shell can improve the surface area and number of active sites, thus improving the kinetics as energy storage devices. As a result, among BCHCSs, BHCSs, and HCSs, BCHCSs exhibit optimal electrochemical performance. The lithium-ion hybrid capacitors employing BCHCSs as an anode can deliver an energy density of 0.2182 kW h kg^-1 at a power density of 0.2235 kW kg^-1. Overall, this study provides an innovative design and strategy for constructing unique carbon nano-architectures for energy storage.

From Micropores to Ultra-micropores inside Hard Carbon: Toward Enhanced Capacity in Room-/Low-Temperature Sodium-Ion Storage

HIGHLIGHTS

Hard-carbon anode dominated with ultra-micropores (< 0.5 nm) was synthesized for sodium-ion batteries via a molten diffusion-carbonization method. The ultra-micropores dominated carbon anode displays an enhanced capacity, which originates from the extra sodium-ion storage sites of the designed ultra-micropores. The thick electrode (~ 19 mg cm-2) with a high areal capacity of 6.14 mAh cm-2 displays an ultrahigh cycling stability and an outstanding low-temperature performance. Pore structure of hard carbon has a fundamental influence on the electrochemical properties in sodium-ion batteries (SIBs). Ultra-micropores (< 0.5 nm) of hard carbon can function as ionic sieves to reduce the diffusion of slovated Na+ but allow the entrance of naked Na+ into the pores, which can reduce the interficial contact between the electrolyte and the inner pores without sacrificing the fast diffusion kinetics. Herein, a molten diffusion-carbonization method is proposed to transform the micropores (> 1 nm) inside carbon into ultra-micropores (< 0.5 nm). Consequently, the designed carbon anode displays an enhanced capacity of 346 mAh g-1 at 30 mA g-1 with a high ICE value of ~ 80.6% and most of the capacity (~ 90%) is below 1 V. Moreover, the high-loading electrode (~ 19 mg cm-2) exhibits a good temperature endurance with a high areal capacity of 6.14 mAh cm-2 at 25 °C and 5.32 mAh cm-2 at - 20 °C. Based on the in situ X-ray diffraction and ex situ solid-state nuclear magnetic resonance results, the designed ultra-micropores provide the extra Na+ storage sites, which mainly contributes to the enhanced capacity. This proposed strategy shows a good potential for the development of high-performance SIBs.

Boosting the lithium and sodium storage performance of graphene-based composite via pore engineering and surface protection

Transition metal oxides with high theoretical capacities are widely investigated as potential anodes for alkali-metal ion batteries. However, the intrinsic conductivity deficiency and large volume changes during cycles result in poor cycling stability and low rate capabilities. Graphene has been widely used to support metal oxide for enhanced performance, but the cycling life is limited by the aggregation/collapse of active materials on graphene surface. Herein, we significantly improve the battery performance of graphene-metal oxide composite via pore engineering and surface protection. In this architecture, the mesoporous NiFe₂O₄ is designed for fast ion diffusion and volume accommodation, and the outer graphene protection can further enhance the electrical conductivity and prevent the aggregation during cycle. Thus, as-prepared G@p-NiFe₂O₄@G composite for lithium storage delivers high capacity (1244 mA h g^-1 after 300 cycles at 0.2 A g^-1), excellent rate performance (563 mA h g^-1 at 4 A g^-1), and outstanding cycling life up to 1200 cycles at 1.5 A g^-1. For sodium storage, it also displays good cycling stability and superior rate performance. Moreover, the effects of various microstructures on the battery performance, the reaction kinetics of various electrodes, and the reaction mechanism of NiFe₂O₄ have been systematically investigated in this work.

Iron Selenide Microcapsules as Universal Conversion-Typed Anodes for Alkali Metal-Ion Batteries

Rechargeable alkali metal-ion batteries (AMIBs) are receiving significant attention owing to their high energy density and low weight. The performance of AMIBs is highly dependent on the electrode materials. It is, therefore, quite crucial to explore suitable electrode materials that can fulfil the future requirements of AMIBs. Herein, a hierarchical hybrid yolk-shell structure of carbon-coated iron selenide microcapsules (FeSe₂ @C-3 MCs) is prepared via facile hydrothermal reaction, carbon-coating, HCl solution etching, and then selenization treatment. When used as the conversion-typed anode materials (CTAMs) for AMIBs, the yolk-shell FeSe₂ @C-3 MCs show advantages. First, the interconnected external carbon shell improves the mechanical strength of electrodes and accelerates ionic migration and electron transmission. Second, the internal electroactive FeSe₂ nanoparticles effectively decrease the extent of volume expansion and avoid pulverization when compared with micro-sized solid FeSe₂ . Third, the yolk-shell structure provides sufficient inner void to ensure electrolyte infiltration and mobilize the surface and near-surface reactions of electroactive FeSe₂ with alkali metal ions. Consequently, the designed yolk-shell FeSe₂ @C-3 MCs demonstrate enhanced electrochemical performance in lithium-ion batteries, sodium-ion batteries, and potassium-ion batteries with high specific capacities, long cyclic stability, and outstanding rate capability, presenting potential application as universal anodes for AMIBs.

Heterostructure Engineering of Core-Shelled Sb@Sb2 O3 Encapsulated in 3D N-Doped Carbon Hollow-Spheres for Superior Sodium/Potassium Storage

In this work, the core-shelled Sb@Sb₂ O₃ heterostructure encapsulated in 3D N-doped carbon hollow-spheres is fabricated by spray-drying combined with heat treatment. The novel core-shelled heterostructures of Sb@Sb₂ O₃ possess a mass of heterointerfaces, which formed spontaneously at the core-shell contact via annealing oxidation and can promote the rapid Na⁺ /K⁺ transfer. The density functional theory calculations revealed the mechanism and significance of Na/K-storage for the core-shelled Sb@Sb₂ O₃ heterostructure, which validated that the coupling between the high-conductivity of Sb and the stability of Sb₂ O₃ can relieve the shortcomings of the individual building blocks, thereby enhancing the Na/K-storage capacity. Furthermore, the core-shell structure embedded in the 3D carbon framework with robust structure can further increase the electrode mechanical strength and thus buffer the severe volume changes upon cycling. As a result, such composite architecture exhibited a high specific capacity of ≈573 mA h g^-1 for sodium-ion battery (SIB) anode and ≈474 mA h g^-1 for potassium-ion battery (PIB) anode at 100 mA g^-1 , and superior rate performance (302 mA h g^-1 at 30 A g^-1 for SIB anode, while 239 mA h g^-1 at 5 A g^-1 for PIB anode).

Antimony nanocrystals self-encapsulated within bio-oil derived carbon for ultra-stable sodium storage

Integrating carbon-coating and nanostructuring has been considered as the most promising strategy to accommodate the dramatic volume expansion represented by high-capacity antimony (Sb) upon sodiation. Suitable coating source and synthetic strategy that are both economical and strong are yet to be explored. In this regard, by using renewable bio-oil as carbon source and self-wrapping precursor, robust Sb@C composite anode with Sb nanoparticles homogeneously impregnated into the cross-linked 2D ultrathin carbon nanosheets is developed via a facile NaCl template-assisted self-assembly and followed carbothermal reduction method. Such judiciously crafted interconnected macroporous framework can mitigate of mechanical stress and alleviate the volume change of inner Sb, guaranteeing high-performance sodium-ion battery anode. At a current density of 0.1 A g^-1, ultrahigh reversible capacity of 520 mAh g^-1 can be achieved. Notably, a stable capacity of 391 mAh g^-1 is even retained after 500 cycles at 1 A g^-1. Such a facile and cost-effective synthetic method is promising for high-performance sodium-ion batteries.

Manipulating Particle Chemistry for Hollow Carbon-based Nanospheres: Synthesis Strategies, Mechanistic Insights, and Electrochemical Applications

Hollow carbon-based nanospheres (HCNs) have been demonstrated to show promising potential in a large variety of research fields, particularly electrochemical devices for energy conversion/storage. The current synthetic protocols for HCNs largely rely on template-based routes (TBRs), which are conceptually straightforward in creating hollow structures but challenged by the time-consuming operations with a low yield in product as well as serious environmental concerns caused by hazardous etching agents. Meanwhile, they showed inadequate ability to build complex carbon-related architectures. Innovative strategies for HCNs free from extra templates thus are highly desirable and are expected to not only ensure precise control of the key structural parameters of hollow architectures with designated functionalities, but also be environmentally benign and scalable approaches suited for their practical applications.In this Account, we outline our recent research progress on the development of template-free protocols for the creation of HCNs with a focus on the acquired mechanical insight into the hollowing mechanism when no extra templates were involved. We demonstrated that carbon-based particles themselves could act as versatile platforms to create hollow architectures through an effective modulation of their inner chemistry. By means of reaction control, the precursor particles were synthesized into solid ones with a well-designed inhomogeneity inside in the form of different chemical parameters such as molecular weight, crystallization degree, and chemical reactivity, by which we not only can create hollow structures inside particles but also have the ability to tune the key features including compositions, porosity, and dimensional architectures. Accordingly, the functionalities of the prepared HCNs could be systematically altered or optimized for their applications. Importantly, the discussed synthesis approaches are facile and environmentally benign processes with potential for scale-up production.The nanoengineering of HNCs is found to be of special importance for their application in a large variety of electrochemical energy storage and conversion systems where the charge transfer and structural stability become a serious concern. Particular attention in this Account is therefore directed to the potential of HCNs in battery systems such as sodium ion batteries (NIBs) and potassium ion batteries (KIBs), whose electrochemical performances are plagued by the destructive volumetric deformation and sluggish charge diffusion during the intercalation/deintercalation of large-size Na⁺ or K⁺. We demonstrated that precise control of the multidimensional factors of the HCNs is critical to offer an optimized design of sufficient reactive sites, excellent charge and mass transport kinetics, and resilient electrode structure and also provide a model system suitable for the study of complicated metal-ion storage mechanisms, such as Na⁺ storage in a hard carbon anode. We expect that this Account will spark new endeavors in the development of HCNs for various applications including energy conversion and storage, catalysis, biomedicine, and adsorption.

Bi-Based Electrode Materials for Alkali Metal-Ion Batteries

Alkali metal (Li, Na, K) ion batteries with high energy density are urgently required for large-scale energy storage applications while the lack of advanced anode materials restricts their development. Recently, Bi-based materials have been recognized as promising electrode candidates for alkali metal-ion batteries due to their high volumetric capacity and suitable operating potential. Herein, the latest progress of Bi-based electrode materials for alkali metal-ion batteries is summarized, mainly focusing on synthesis strategies, structural features, storage mechanisms, and the corresponding electrochemical performance. Particularly, the optimization of electrode-electrolyte interphase is also discussed. In addition, the remaining challenges and further perspectives of Bi-based electrode materials are outlined. This review aims to provide comprehensive knowledge of Bi-based materials and offer a guideline toward more applications in high-performance batteries.

Rational Design of Sb@C@TiO2 Triple-Shell Nanoboxes for High-Performance Sodium-Ion Batteries

Antimony is an attractive anode material for sodium-ion batteries (SIBs) owing to its high theoretical capacity and appropriate sodiation potential. However, its practical application is severely impeded by its poor cycling stability caused by dramatic volumetric variations during sodium uptake and release processes. Here, to circumvent this obstacle, Sb@C@TiO₂ triple-shell nanoboxes (TSNBs) are synthesized through a template-engaged galvanic replacement approach. The TSNB structure consists of an inner Sb hollow nanobox protected by a conductive carbon middle shell and a TiO₂ -nanosheet-constructed outer shell. This structure offers dual protection to the inner Sb and enough room to accommodate volume expansion, thus promoting the structural integrity of the electrode and the formation of a stable solid-electrolyte interface film. Benefiting from the rational structural design and synergistic effects of Sb, carbon, and TiO₂ , the Sb@C@TiO₂ electrode exhibits superior rate performance (212 mAh g^-1 at 10 A g^-1 ) and outstanding long-term cycling stability (193 mAh g^-1 at 1 A g^-1 after 4000 cycles). Moreover, a full cell assembled with a configuration of Sb@C@TiO₂ //Na₃ (VOPO₄ )₂ F displays a high output voltage of 2.8 V and a high energy density of 179 Wh kg^-1 , revealing the great promise of Sb@C@TiO₂ TSNBs as the electrode in SIBs.

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.

Novel Approach Through the Harmonized Sulfur in Disordered Carbon Structure for High-Efficiency Sodium-Ion Exchange

Sodium-ion batteries (SiBs) have recently attracted considerable interest due to the plentiful supply of raw materials for their production and their electrochemical behavior, which is similar to that of lithium-ion batteries (LiBs). However, the relatively larger radius of sodium ions than that of lithium ions is not suitable for storage in conventional graphite, which is widely used as the anode. To resolve this issue, in this study, we developed a new harmonized carbon material with a three-dimensional (3D) grapevine-like structure and a sulfur component using an efficient synthesis process. On the basis of these advantages, the harmonized sulfur-carbon material exhibited a highly reversible capacity of 146 mA h g^-1 at an extremely high specific current of 100 A g^-1 and long-term galvanostatic cycling stability at 10 and 100 A g^-1 with superior electrochemical performance. Our results are anticipated to provide new insights into SiB anode materials that would advance their production.