Hierarchical Porous Nanosheets Constructed by Graphene-Coated, Interconnected TiO2 Nanoparticles for Ultrafast Sodium Storage

Hierarchical Design of Mn2P Nanoparticles Embedded in N,P-Codoped Porous Carbon Nanosheets Enables Highly Durable Lithium Storage

Although transition metal phosphide anodes possess high theoretical capacities, their inferior electronic conductivities and drastic volume variations during cycling lead to poor rate capability and rapid capacity fading. To simultaneously overcome these issues, we report a hierarchical heterostructure consisting of isolated Mn₂P nanoparticles embedded into nitrogen- and phosphorus-codoped porous carbon nanosheets (denoted as Mn₂P@NPC) as a viable anode for lithium-ion batteries (LIBs). The resulting Mn₂P@NPC design manifests outstanding electrochemical performances, namely, high reversible capacity (598 mA h g^-1 after 300 cycles at 0.1 A g^-1 ), exceptional rate capability (347 mA h g^-1 at 4 A g^-1), and excellent cycling stability (99% capacity retention at 4 A g^-1 after 2000 cycles). The robust structure stability of Mn₂P@NPC electrode during cycling has been revealed by the in situ and ex situ transmission electron microscopy (TEM) characterizations, giving rise to long-term cyclability. Using in situ selected area electron diffraction and ex situ high-resolution TEM studies, we have unraveled the dominant lithium storage mechanism and confirmed that the superior lithium storage performance of Mn₂P@NPC originated from the reversible conversion reaction. Furthermore, the prelithiated Mn₂P@NPC∥LiFePO₄ full cell exhibits impressive rate capability and cycling stability. This work introduces the potential for engineering high-performance anodes for next-generation high-energy-density LIBs.

Systematic Study of Alkali Cations Intercalated Titanium Dioxide Effect on Sodium and Lithium Storage

The fast development of electrochemical energy storage devices necessitates rational design of the high-performance electrode materials and systematic and deep understanding of the intrinsic energy storage processes. Herein, the preintercalation general strategy of alkali ions (A = Li⁺ , Na⁺ , K⁺ ) into titanium dioxide (A-TO, LTO, NTO, KTO) is proposed to improve the structural stability of anode materials for sodium and lithium storage. The different optimization effects of preintercalated alkali ions on electrochemical properties are studied systematically. Impressively, the three electrode materials manifest totally different capacities and capacity retention. The efficiency of the energy storage process is affected not only by the distinctive structure but also by the suitable interlayer spacing of Ti-O, as well as by the interaction effect between the host Ti-O layer and alien cations with proper size, demonstrating the pivotal role of the sodium ions. The greatly enhanced electrochemical performance confirms the importance of rational engineering and synthesis of advanced electrode materials with the preintercalation of proper alkali cations.

CNT-Assembled Octahedron Carbon-Encapsulated Cu3P/Cu Heterostructure by In Situ MOF-Derived Engineering for Superior Lithium Storage: Investigations by Experimental Implementation and First-Principles Calculation

Conspicuously, metal-organic frameworks (MOFs) serve as homogenously and periodically atom-dispersed self-sacrificial template for in situ engineering of hierarchical porous carbon-encapsulated micro/nanoheterostructure materials, integrating the merits of micro/nanostructure to high-volumetric energy storage. Copper phosphide represents a promising candidate due to its compact material density compared to commercial graphite. Herein, micro/nanostructured Cu3P/Cu encapsulated by carbon-nanotube-assembled hierarchical octahedral carbonaceous matrix (Cu3P/Cu@CNHO) is constructed by an in situ MOF-derived engineering for novel anode material in LIBs, which achieves an extraordinary cycling stability (a well-maintained gravimetric/volumetric capacity of 463.2 mAh g-1/1878.4 mAh cm-3 at 1 A g-1 up to 1600 cycles) and distinguished rate capability (an ameliorated capacity of 317.7 mAh g-1 even at 10 A g-1), together with unprecedented heat-resistant capability (an elevated temperature of 50 °C for 1000 cycles maintaining 434.7 mAh g-1 at 0.5 A g-1). The superior electrochemical performance of Cu3P/Cu@CNHO is credited to the large specific surface area, conductive carbon matrix and metallic copper dopants, synergistic effects of the intrinsic Cu3P/Cu heterostructure, and well-defined micro/nanostructure, facilitating a boosted electrochemical conductivity and accelerated diffusion kinetics.

MXene-Derived Defect-Rich TiO2@rGO as High-Rate Anodes for Full Na Ion Batteries and Capacitors

Sodium ion batteries and capacitors have demonstrated their potential applications for next-generation low-cost energy storage devices. These devices's rate ability is determined by the fast sodium ion storage behavior in electrode materials. Herein, a defective TiO2@reduced graphene oxide (M-TiO2@rGO) self-supporting foam electrode is constructed via a facile MXene decomposition and graphene oxide self-assembling process. The employment of the MXene parent phase exhibits distinctive advantages, enabling defect engineering, nanoengineering, and fluorine-doped metal oxides. As a result, the M-TiO2@rGO electrode shows a pseudocapacitance-dominated hybrid sodium storage mechanism. The pseudocapacitance-dominated process leads to high capacity, remarkable rate ability, and superior cycling performance. Significantly, an M-TiO2@rGO//Na3V2(PO4)3 sodium full cell and an M-TiO2@rGO//HPAC sodium ion capacitor are fabricated to demonstrate the promising application of M-TiO2@rGO. The sodium ion battery presents a capacity of 177.1 mAh g-1 at 500 mA g-1 and capacity retention of 74% after 200 cycles. The sodium ion capacitor delivers a maximum energy density of 101.2 Wh kg-1 and a maximum power density of 10,103.7 W kg-1. At 1.0 A g-1, it displays an energy retention of 84.7% after 10,000 cycles.

Hierarchical Octahedra Constructed by Cu2 S/MoS2 ⊂Carbon Framework with Enhanced Sodium Storage

Metal sulfides have aroused considerable attention for efficient sodium storage because of their high capacity and decent redox reversibility. However, the poor rate capability and fast capacity decay greatly hinder their practical application in sodium-ion batteries. Herein, a self-template-based strategy is designed to controllably synthesize hierarchical microoctahedra assembled with Cu₂ S/MoS₂ heterojunction nanosheets in the porous carbon framework (Cu₂ S/MoS₂ ⊂PCF) via a facile coprecipitation method coupled with vulcanization treatment. The Cu₂ S/MoS₂ ⊂PCF microoctahedra with 2D hybrid nanosubunits reasonably integrate several merits including facilitating the diffusion of electrons and Na⁺ ions, enhancing the electric conductivity, accelerating the ion and charge transfer, and buffering the volume variation. Therefore, the Cu₂ S/MoS₂ ⊂PCF composite manifests efficient sodium storage performance with high capacity, long cycling life, and excellent rate capability.

High Pseudocapacitance Boosts Ultrafast, High-Capacity Sodium Storage of 3D Graphene Foam-Encapsulated TiO2 Architecture

Anatase TiO₂ is an attractive anode for Li-ion batteries and Na-ion batteries because of its structural stability. However, the electrochemical capability of anatase TiO₂ is unsatisfactory due to its intrinsically low electrical conductivity and poor ion diffusivity at the electrode/electrolyte interface. We prepared 3D lightweight graphene aerogel-encapsulated anatase TiO₂, which exhibits a high reversible capacity (390 mA h g^-1 at 50 mA g^-1), a superior rate performance (164.9 mA h g^-1 at 5 A g^-1), and a long-term cycling capability (capacity retention of 86.8% after 7800 cycles). The major energy-storage mechanism is surface capacitance dominated, which favors a high capacity and fast Na⁺ uptake. The inherent features of 3D porous aerogels provide additional active reaction sites and facilitate fast charge diffusion and easy ion access. This will enable the development of 3D interconnected, graphene-based, high-capacity active materials for the development of next-generation energy-storage applications.

Bio-derived yellow porous TiO2: the lithiation induced activation of an oxygen-vacancy dominated TiO2 lattice evoking a large boost in lithium storage performance

Oxygen deficient TiO₂ has attracted extensive attention owning to its narrow bandgap and high electrical conductivity. In this work, novel yellow TiO₂ with hierarchically porous architecture is fabricated by a facile pyrolysis method in air via a biomass template. The obtained yellow TiO₂ exhibits interesting lithiation induced activation during cycling, which gives rise to a phase change from poorly crystallized TiO₂ to an amorphous phase, accompanied by a colour change from yellow to black. In contrast to the intercalation mechanism reported in most of the literature on the TiO₂ anode of LIBs, notably, the reversible redox reaction between Ti³⁺ and metal Ti can be verified in this case, demonstrating the novel conversion reaction mechanism of the TiO₂ electrode. Based on this, the yellow porous TiO₂ delivers enhanced electrochemical performance as an anode for LIBs with a superior capacity of 480 mA h g^-1 at 5 A g^-1 and a high capacity of 206 mA h g^-1 at 10 A g^-1 after 8000 cycles.

Fabrication of 2D/2D nanosheet heterostructures of ZIF-derived Co3S4 and g-C3N4 for asymmetric supercapacitors with superior cycling stability

Asymmetric supercapacitors with superior cycling stability are achieved by designing 2D/2D nanosheet heterostructures of ZIF-derived Co₃S₄ and g-C₃N₄.

Electrospinning-based construction of porous Mn3O4/CNFs as anodes for high-performance lithium-ion batteries

Porous one-dimensional Mn₃O₄/CNFs composites are fabricated and used as anode materials for Li-ion batteries; they exhibit excellent electrochemical performance.

Comparison of Charge Storage Properties of Prussian Blue Analogues Containing Cobalt and Copper

Prussian blue analogues are of great interest as alternative battery materials because of their long life cycle and potential use of earth-abundant elements. In this work, thin film mixed-metal hexacyanoferrates (HCFs) based on NiCo and NiCu alloys were fabricated in an all electrochemical process. The structure and composition of the samples were characterized, along with the charge storage capacity and kinetics of the charge transfer reaction. For both NiCo-HCF and NiCu-HCF samples, the total charge capacity increased with the substitution of Ni with more Co or Cu, and the increase was larger for Cu samples than for Co samples. On the other hand, the charge storage kinetics had only a modest change with substituted metal, and these effects were independent of the amount of that substitution. Thus, the mixed-metal HCFs have promise for increasing overall storage capacity without negatively influencing the rate capability when used in battery applications.

One-Step Construction of MoS0.74Se1.26/N-Doped Carbon Flower-like Hierarchical Microspheres with Enhanced Sodium Storage

Despite the fulfilling advancement in preparing two-dimensional (2D) layered transition-metal dichalcogenide (TMD)-based hybrid architectures, most methods lie on additional template-based procedures for obtaining the expected structure. Here, we present a self-template and in situ synchronous selenization/vulcanization strategy for the synthesis of flower-like hierarchical MoS_0.74Se_1.26/N-doped carbon (MoS_0.74Se_1.26/NC) microspheres by morphology-preserved thermal transformation of a Mo-polydopamine precursor. Introducing element S into the MoSe₂ crystal structure can enhance the electron and ion transportation and lift the ability of MoSe₂ to store Na⁺; the presence of Se can expand the interlayer spacing in contrast to MoS₂. Moreover, carbon composition can also favor the electrical conductivity, and the rigid micro/nanostructure is better for preventing the stacking of MoS_0.74Se_1.26 nanoflakes. The Mo-polydopamine-derived MoS_0.74Se_1.26/NC hybrid exhibits much better performance as the sodium-ion battery (SIB) anode than MoS₂/NC and MoSe₂/NC counterparts, confirming that the advanced electrode material can be attained via the rationalization of the synthetic method. The work provides a new design configuration for novel 2D layered TMDs in the promising application of SIBs as anode materials.

Heterostructured TiO2 Spheres with Tunable Interiors and Shells toward Improved Packing Density and Pseudocapacitive Sodium Storage

Insertion-type anode materials with beneficial micro- and nanostructures are proved to be promising for high-performance electrochemical metal ion storage. In this work, heterostructured TiO₂ shperes with tunable interiors and shells are controllably fabricated through newly proposed programs, resulting in enhanced pseudocapacitive response as well as favorable Na⁺ storage kinetics and performances. In addition, reasonably designed nanosheets in the extrinsic shells are also able to reduce the excess space generated by hierarchical structure, thus improving the packing density of TiO₂ shperes. Lastly, detailed density functional theory calculations with regard to sodium intercalation and diffusion in TiO₂ crystal units are also employed, further proving the significance of the surface-controlled pseudocapacitive Na⁺ storage mechanism. The structure design strategies and experimental results demonstrated in this work are meaningful for electrode material preparation with high rate performance and volume energy density.

Enabling Superior Electrochemical Properties for Highly Efficient Potassium Storage by Impregnating Ultrafine Sb Nanocrystals within Nanochannel-Containing Carbon Nanofibers

Sb-based nanocomposites are attractive anode materials for batteries as they exhibit large theoretical capacity and impressive working voltage. However, tardy potassium ion diffusion characteristics, unstable Sb/electrolyte interphase, and huge volume variation pose a challenge, hindering their practical use for potassium-ion batteries (PIBs). Now, a simple robust strategy is presented for uniformly impregnating ultrasmall Sb nanocrystals within carbon nanofibers containing an array of hollow nanochannels (denoted u-Sb@CNFs), resolving the issues above and yielding high-performance PIBs. u-Sb@CNFs can be directly employed as an anode, thereby dispensing with the need for conductive additives and binders. Such a judiciously crafted u-Sb@CNF-based anode renders a set of intriguing electrochemical properties, representing large charge capacity, unprecedented cycling stability, and outstanding rate performance. A reversible capacity of 225 mAh g^-1 is retained after 2000 cycles at 1 A g^-1 .

Defect-Assisted Selective Surface Phosphorus Doping to Enhance Rate Capability of Titanium Dioxide for Sodium Ion Batteries

Phosphorus doping is an effective strategy to simultaneously improve the electronic conductivity and regulate the ionic diffusion kinetics of TiO₂ being considered as anode materials for sodium ion batteries. However, efficient phosphorus doping at high concentration in well-crystallized TiO₂ nanoparticles is still a big challenge. Herein, we propose a defect-assisted phosphorus doping strategy to selectively engineer the surface structure of TiO₂ nanoparticles. The reduced TiO_2-x shell layer that is rich in oxygen defects and Ti³⁺ species precisely triggered a high concentration of phosphorus doping (∼7.8 at. %), and consequently a TiO₂@TiO_2-x-P core@shell architecture was produced. Comprehensive characterizations and first-principle calculations proved that the surface-functionalized TiO_2-x-P thin layer endowed the TiO₂@TiO_2-x-P with substantially enhanced electronic conductivity and accelerated Na ion transportation, resulting in great rate capability (167 mA h g^-1 at 10 000 mA g^-1) and stable cycling (99% after 5000 cycles at 10 A g^-1). Combining in/ex situ X-ray diffraction with ex situ electron spin resonance clearly demonstrated the high reversibility and robust mechanical behavior of TiO₂@TiO_2-x-P upon long-term cycling. This work provides an interesting and effective strategy for precise heteroatoms doping to improve the electrochemical performance of nanoparticles.

Anatase TiO2 as a Na+-Storage Anode Active Material for Dual-Ion Batteries

Anatase TiO₂ used as the sodium-storage anode is coupled with a graphite cathode to construct dual-ion batteries. The batteries display wide voltage window (1.0-4.7 V), long cycling stability (98 mAh g^-1 after 1400 cycles at 500 mA g^-1), and considerable rate performance (102 mAh g^-1 at 1500 mA g^-1). Furthermore, the kinetic behaviors of Na⁺ and PF₆^- ions are investigated through electrochemical impedance spectroscopy, the galvanostatic intermittent titration technique, and the potentiostatic intermittent titration technique. The corresponding apparent ion diffusion coefficient is calculated, and the results indicate that the transport of PF₆^- anions in the graphite cathode is swift.

Rational Design of Layered SnS2 on Ultralight Graphene Fiber Fabrics as Binder-Free Anodes for Enhanced Practical Capacity of Sodium-Ion Batteries

Generally, the practical capacity of an electrode should include the weight of non-active components such as current collector, polymer binder, and conductive additives, which were as high as 70 wt% in current reported works, seriously limiting the practical capacity. This work pioneered the usage of ultralight reduced graphene fiber (rGF) fabrics as conductive scaffolds, aiming to reduce the weight of non-active components and enhance the practical capacity. Ultrathin SnS2 nanosheets/rGF hybrids were prepared and used as binder-free electrodes of sodium-ion batteries (SIBs). The interfused graphene fibers endow the electrode a porous, continuous, and conductive network. The in situ phase transformation from SnO2 to SnS2 could preserve the strong interfacial interactions between SnS2 and graphene. Benefitting from these, the designed binder-free electrode delivers a high specific capacity of 500 mAh g-1 after 500 cycles at a current rate of 0.5 A g-1 with almost 100% Coulombic efficiency. Furthermore, the weight percentage of SnS2 in the whole electrode could reach up to 67.2 wt%, much higher than that of common electrode configurations using Cu foil, Al foil, or carbon cloth, significantly highlighting the ultralight characters and advantages of the rGF fabrics for using as binder-free electrodes of SIBs.

In Situ Fabrication of Branched TiO2 /C Nanofibers as Binder-Free and Free-Standing Anodes for High-Performance Sodium-Ion Batteries

Herein, 1D free-standing and binder-free hierarchically branched TiO₂ /C nanofibers (denoted as BT/C NFs) based on an in situ fabrication method as an anode for sodium-ion batteries are reported. The in situ fabrication endows this material with large surface area and strong structural stability, providing this material with abundant active sites and smooth channels for fast ion transportation. As a result, BT/C NFs with the character of free-standing membranes are directly used as binder-free anode for sodium-ion batteries, delivering a capacity of 284 mA h g^-1 at a current density of 200 mA g^-1 after 1000 cycles. Even at a high current density of 2000 mA g^-1 , the reversible capacity can still achieve as high as 204 mA h g^-1 . By means of kinetic analysis, it is demonstrated that the remarkable surface pseudocapacitive behavior is also a major factor to achieve excellent performance. The rationally designed structure coupled with the inherent pseudocapacitive behavior gives this material potential for sodium-ion batteries.

Unusual formation of NiCo2O4@MnO2/nickel foam/MnO2 sandwich as advanced electrodes for hybrid supercapacitors

A facile three-step method is designed for large-scale preparation of a NiCo₂O₄@MnO₂/nickel foam/MnO₂ sandwich architecture with robust adhesion as an advanced electrode for high-performance supercapacitors. The synthesis contains the hydrothermal reaction of a cobalt-nickel hydroxide precursor on a nickel foam (NF) support and subsequent thermal conversion into spinel mesoporous NiCo₂O₄ nanowire arrays, followed by a hydrothermal oxidation reaction to synthesize NiCo₂O₄@MnO₂/nickel foam/MnO₂ sandwiches. Moreover, the tactics reported in this study enable easy control of the growth of NiCo₂O₄ on one side of the NF and MnO₂ nanosheets on both sides of the NF to obtain novel NiCo₂O₄@MnO₂/nickel foam/MnO₂ sandwiches. Because of the unusual structural and compositional features, the obtained NiCo₂O₄@MnO₂/nickel foam/MnO₂ sandwiches manifest excellent performance with high specific capacitance (1.70 C cm^-2 at 2 mA cm^-2), exceptional rate capability (78.5% retention at 20 mA cm^-2) and ultralong cycling stability (91% retention over 30 000 cycles at 20 mA cm^-2) as a battery-type electrode material for supercapacitors. When further assembled into an aqueous hybrid supercapacitor, it can deliver an energy density of 53.5 W h kg^-1 at a power density of 80 W kg^-1 and 20.7 W h kg^-1 at 8 kW kg^-1. This novel sandwich electrode provides a new idea for improving the electrochemical performance of hybrid supercapacitors.

Pseudocapacitive Na+ Insertion in Ti-O-C Channels of TiO2-C Nanofibers with High Rate and Ultrastable Performance

Ti-O-C channels for ultrafast sodium storage were constructed in N/S-co-doped TiO₂-C nanofibers, which deliver a high rate performance of 181.9 mAh g^-1 at 5 A g^-1 after 3000 cycles. The existence of Ti-O-C bonds at the interface of TiO₂-C phases was revealed by synchrotron radiation X-ray absorption spectra. Based on this, first-principles calculations further verified the low energy barrier for Na⁺ insertion/extraction in the Ti-O-C channels formed by the intimately integrated graphite layer with TiO₂ near the surface. In addition, surface defects induced by heteroatoms accelerate the Na⁺ mass transfer through the pathway from the carbon surface to the Ti-O-C channel.

Mesopore-Induced Ultrafast Na+ -Storage in T-Nb2 O5 /Carbon Nanofiber Films toward Flexible High-Power Na-Ion Capacitors

Hybrid Na-ion capacitors (NICs) are receiving considerable interest because they combine the merits of both batteries and supercapacitors and because of the low-cost of sodium resources. However, further large-scale deployment of NICs is impeded by the sluggish diffusion of Na⁺ in the anode. To achieve rapid redox kinetics, herein the controlled fabrication of mesoporous orthorhombic-Nb₂ O₅ (T-Nb₂ O₅ )/carbon nanofiber (CNF) networks is demonstrated via in situ SiO₂ -etching. The as-obtained mesoporous T-Nb₂ O₅ (m-Nb₂ O₅ )/CNF membranes are mechanically flexible without using any additives, binders, or current collectors. The in situ formed mesopores can efficiently increase Na⁺ -storage performances of the m-Nb₂ O₅ /CNF electrode, such as excellent rate capability (up to 150 C) and outstanding cyclability (94% retention after 10 000 cycles at 100 C). A flexible NIC device based on the m-Nb₂ O₅ /CNF anode and the graphene framework (GF)/mesoporous carbon nanofiber (mCNF) cathode, is further constructed, and delivers an ultrahigh power density of 60 kW kg^-1 at 55 Wh kg^-1 (based on the total weight of m-Nb₂ O₅ /CNF and GF/mCNF). More importantly, owing to the free-standing flexible electrode configuration, the m-Nb₂ O₅ /CNF//GF/mCNF NIC exhibits high volumetric energy and power densities (11.2 mWh cm^-3 , 5.4 W cm^-3 ) based on the full device, which holds great promise in a wide variety of flexible electronics.

Hollow Multishelled Heterostructured Anatase/TiO2 (B) with Superior Rate Capability and Cycling Performance

TiO₂ is a potential anode material for lithium-ion batteries due to its high rate capability and high safety. Here, a controllable synthesis for hollow nanostructured TiO₂ , with heterostructured shells of TiO₂ (B) and anatase phases, is presented for the first time, by using a sequential templating approach. The hollow nanostructures can be easily controlled to produce core-shell and double-shelled materials with different compositional ratios of anatase to TiO₂ (B) by tuning the synthetic conditions. When used as the anode materials for lithium-ion batteries, a specific discharge capacity of 215.4 mAh g^-1 for the double-shelled anatase/TiO₂ (B) hollow microspheres is achieved at a current rate of 1 C (335 mA g^-1 ) for the 100th cycle and shows high specific discharge capacities of 141.6 and 125.7 mAh g^-1 at the high rates of 10 and 20 C over 1000 cycles. These results are due to the unique stable hollow multishelled structure, which has a high specific surface area, as well as the interface between the heterostructured anatase/TiO₂ (B) phases contributing a substantial number of lithium-ion storage sites.

New Insights into the Electrochemistry Superiority of Liquid Na-K Alloy in Metal Batteries

The significant issues with alkali metal batteries arise from their poor electrochemical properties and safety problems, limiting their applications. Herein, TiO₂ nanoparticles embedded into N-doped porous carbon truncated ocatahedra (TiO₂ ⊂NPCTO) are engineered as a cathode material with different metal anodes, including solid Na or K and liquid Na-K alloy. Electrochemical performance and kinetics are systematically analyzed, with the aim to determine detailed electrochemistry. By using a galvanostatic intermittent titration technique, TiO₂ ⊂NPCTO/NaK shows faster diffusion of metal ions in insertion and extraction processes than that of Na-ions and K-ions in solid Na and K. The lower reaction resistance of liquid Na-K alloy electrode is also examined. The higher b-value of TiO₂ ⊂NPCTO/NaK confirms that the reaction kinetics are promoted by the surface-induced capacitive behavior, favorable for high rate performance. This superiority highly pertains to the distinct liquid-liquid junction between the electrolyte and electrode, and the prohibition of metal dendrite growth, substantiated by symmetric cell testing, which provides a robust and homogeneous interface more stable than the traditional solid-liquid one. Hence, the liquid Na-K alloy-based battery exhibits to better cyclablity with higher capacity, rate capability, and initial coulombic efficiency than solid Na and K batteries.

Recent Progress on Molybdenum Oxides for Rechargeable Batteries

Diminishing fossil-fuel resources and a rise in energy demands has required the pursuit of sustainable and rechargeable energy-storage materials, including batteries and supercapacitors, the electrochemical properties of which depend largely on the electrode materials. In recent decades, numerous electrode materials with excellent electrochemical energy-storage capabilities, long life spans, and environmentally acceptable qualities have been developed. Among existing materials, molybdenum oxides containing MoO₃ and MoO₂ , as well as their composites, are very fascinating contenders for competent energy-storage devices because of their exceptional physicochemical properties, such as thermal stability, high theoretical capability, and mechanical strength. This Minireview mainly focuses on the latest progress for the use of molybdenum oxides as electrode materials for lithium-ion batteries; sodium-ion batteries; and other novel batteries, such as lithium-sulfur batteries, lithium-oxygen batteries, and newly developed hydrogen-ion batteries, with a focus on studies of the reaction mechanism, design of the electrode structures, and improvement of the electrochemical properties.

Coaxial Carbon Nanotube Supported TiO2@MoO2@Carbon Core-Shell Anode for Ultrafast and High-Capacity Sodium Ion Storage

The sluggish kinetic in electrode materials is one of the critical challenges in achieving high-power sodium ion storage. We report a coaxial core-shell nanostructure composed of carbon nanotube (CNT) as the core and TiO₂@MoO₂@C as shells for a hierarchically nanoarchitectured anode for improved electrode kinetics. The 1D tubular nanostructure can effectively reduce ion diffusion path, increase electrical conductivity, accommodate the stress due to volume change upon cycling, and provide additional interfacial active sites for enhanced charge storage and transport properties. Significantly, a synergistic effect between TiO₂ and MoO₂ nanostructures is investigated through ex situ solid-state nuclear magnetic resonance. The electrode exhibits a good rate capability (150 mAh g^-1 at 20 A g^-1) and superior cycling stability with a reversibly capacity of 175 mAh g^-1 at 10 A g^-1 for over 8000 cycles.

TiO2-Coated Interlayer-Expanded MoSe2/Phosphorus-Doped Carbon Nanospheres for Ultrafast and Ultralong Cycling Sodium Storage

Based on multielectron conversion reactions, layered transition metal dichalcogenides are considered promising electrode materials for sodium-ion batteries, but suffer from poor cycling performance and rate capability due to their low intrinsic conductivity and severe volume variations. Here, interlayer-expanded MoSe2/phosphorus-doped carbon hybrid nanospheres coated by anatase TiO2 (denoted as MoSe2/P-C@TiO2) are prepared by a facile hydrolysis reaction, in which TiO2 coating polypyrrole-phosphomolybdic acid is utilized as a novel precursor followed by a selenization process. Benefiting from synergistic effects of MoSe2, phosphorus-doped carbon, and TiO2, the hybrid nanospheres manifest unprecedented cycling stability and ultrafast pseudocapacitive sodium storage capability. The MoSe2/P-C@TiO2 delivers decent reversible capacities of 214 mAh g-1 at 5.0 A g-1 for 8000 cycles, 154 mAh g-1 at 10.0 A g-1 for 10000 cycles, and an exceptional rate capability up to 20.0 A g-1 with a capacity of ≈175 mAh g-1 in a voltage range of 0.5-3.0 V. Coupled with a Na3V2(PO4)3@C cathode, a full cell successfully confirms a reversible capacity of 242.2 mAh g-1 at 0.5 A g-1 for 100 cycles with a coulombic efficiency over 99%.

A Salt-Templated Strategy toward Hollow Iron Selenides-Graphitic Carbon Composite Microspheres with Interconnected Multicavities as High-Performance Anode Materials for Sodium-Ion Batteries

In this work, a facile salt-templated approach is developed for the preparation of hollow FeSe₂ /graphitic carbon composite microspheres as sodium-ion battery anodes; these are composed of interconnected multicavities and an enclosed surface in-plane embedded with uniform hollow FeSe₂ nanoparticles. As the precursor, Fe₂ O₃ /carbon microspheres containing NaCl nanocrystals are obtained using one-pot ultrasonic spray pyrolysis in which inexpensive NaCl and dextrin are used as a porogen and carbon source, respectively, enabling mass production of the composites. During post-treatment, Fe₂ O₃ nanoparticles in the composites transform into hollow FeSe₂ nanospheres via the Kirkendall effect. These rational structures provide numerous conductive channels to facilitate ion/electron transport and enhance the capacitive contribution. Moreover, the synergistic effect between the hollow cavities within FeSe₂ and the outstanding mechanical strength of the porous carbon matrix can effectively accommodate the large volume changes during cycling. Correspondingly, the composite microsphere exhibits high discharge capacity of 510 mA h g^-1 after 200 cycles at 0.2 A g^-1 with capacity retention of 88% when calculated from the second cycle. Even at a high current density of 5.0 A g^-1 , a high discharge capacity of 417 mA h g^-1 can be achieved.

A mesoporous C,N-co doped Co-based phosphate ultrathin nanosheet derived from a phosphonate-based-MOF as an efficient electrocatalyst for water oxidation

A mesoporous C,N-co doped Co-based phosphate ultrathin nanosheet derived from 2D phosphate MOFs has been explored and exhibits highly efficient OER performance.

Flexible Freestanding Carbon Nanofiber-Embedded TiO2 Nanoparticles as Anode Material for Sodium-Ion Batteries

Sodium-ion batteries (SIBs), owning to the low cost, abundant resources, and similar physicochemical properties with lithium-ion batteries (LIBs), have earned much attention for large-scale energy storage systems. In this article, we successfully synthesize flexible freestanding carbon nanofiber-embedded TiO₂ nanoparticles (CNF-TiO₂) and then apply it directly as anode in SIBs without binder or current collector. Taking the advantage of flexible CNF and high structural stability, this anode exhibits high reversible capacity of 614 mAh·g^-1 (0.27 mAh·cm^-2) after almost 400 cycles and excellent capacity retention ability of ~100.

1D Sub-Nanotubes with Anatase/Bronze TiO2 Nanocrystal Wall for High-Rate and Long-Life Sodium-Ion Batteries

The development of 1D nanostructures with enhanced material properties has been an attractive endeavor for applications in energy and environmental fields, but it remains a major research challenge. Herein, this work demonstrates a simple, gel-derived method to synthesize uniform 1D elongated sub-nanotubes with an anatase/bronze TiO₂ nanocrystal wall (TiO₂ SNTs). The transformation mechanism of TiO₂ SNTs is studied by various ex situ characterization techniques. The resulting 1D nanostructures exhibit, synchronously, a high aspect ratio, open tubular interior, and anatase/bronze nanocrystal TiO₂ wall. This results in excellent properties of electron/ion transport and reaction kinetics. Consequently, as an anode material for sodium-ion batteries (SIBs), the TiO₂ SNTs display an ultrastable long-life cycling stability with a capacity of 107 mAh g^-1 at 16 C after 4000 cycles and a high-rate capacity of 94 mAh g^-1 at 32 C. This a high-rate and long-life performance is superior to any report on pure TiO₂ for SIBs. This work provides new fundamental information for the design and fabrication of inorganic structures for energy and environmental applications.

A densely packed Sb2O3 nanosheet-graphene aerogel toward advanced sodium-ion batteries

As a promising anodic material for rechargeable batteries, Sb2O3 has drawn increasing attention due to its high theoretical capacity and abundant natural deposits. However, poor cyclability and rate performance of Sb2O3 derived from a large volume change during insertion/desertion reactions as well as a sluggish kinetic process restrict its practical application. Herein, we report a facile amorphous-to-crystalline strategy to synthesize a densely packed Sb2O3 nanosheet-graphene aerogel as a novel anode for sodium ion batteries (SIBs). This Sb2O3/graphene composite displays a reversible capacity as high as 657.9 mA h g-1 even after 100 cycles at 0.1 A g-1, along with an excellent rate capacity of 356.8 mA h g-1 at 5.0 A g-1. The superior electrochemical performance is attributed to the synergistic effects of densely packed Sb2O3 nanosheets and graphene aerogel, which serves as both a robust support and stable buffer layer to maintain the structural stability of the nanocomposite, and enhances the electrode kinetics of electrolyte diffusion and electron transfer simultaneously. Hence, this densely-packed two-dimensional Sb2O3 nanosheet-graphene aerogel can be a promising anode material for rechargeable SIBs due to its facile synthesis process and outstanding electrochemical performance.

Three-Dimensional Hierarchical Framework Assembled by Cobblestone-Like CoSe2@C Nanospheres for Ultrastable Sodium-Ion Storage

Sodium-ion batteries (SIBs), as the promising commercial energy system, are restricted by their sluggish kinetics and low sodium-ion storage. Metal selenide possesses good conductivity and capacity but still suffers from the stacked problem and volume expansion. Significantly, CoSe₂/C is successfully prepared with the assistance of citric acid as both a chelating agent and carbon precursor, displaying that cobblestone-like nanospheres with the radii (<25 nm) distribute uniformly in the carbon matrix. It is expected that the established Co-O-C bonds enhance the stability of the structure with faster ion shuttling. With the available electrolyte (NaCF₃SO₃/diethylene glycol dimethyl ether) in a potential window range from 0.5 to 3.0 V, the as-obtained sample shows the ultralong lifespan at 4.5 A g^-1, retaining a capacity of 345 mA h g^-1 after 10 000 cycles. From the detailed kinetic analysis, it is clear that the surface-controlled electrochemical behavior mainly contributes to the excellent large-current cycling stability and Na storage capacity. The ex situ results support that the crystal and morphological structure remains stable. This work is anticipated to enhance the in-depth understanding of the CoSe₂/C anode and supply a facile manner to obtain electrode materials for SIBs.

Conjugated hybrid films based on a new polyoxotitanate monomer

Electropolymerisation of the novel polyoxotitanate (POT) hexamer [Ti(μ₃-O)(OⁱPr)(TA)]₆ (TA = thiophene-3-acetate) with 3,4-ethylenedioxythiophene (EDOT) gives films of hybrid conjugated copolymer, Poly-(EDOT-POT)s, the morphologies of which are, uniquely, influenced by the electropolymerisation potential.