Synergizing Phase and Cavity in CoMoOx Sy Yolk-Shell Anodes to Co-Enhance Capacity and Rate Capability in Sodium Storage

Pressure-Induced Defects and Reduced Size Endow TiO2 with High Capacity over 20 000 Cycles and Excellent Fast-Charging Performance in Sodium Ion Batteries

Anatase TiO2 as sodium-ion-battery anode has attracted increased attention because of its low volume change and good safety. However, low capacity and poor rate performance caused by low electrical conductivity and slow ion diffusion greatly impede its practical applications. Here, a bi-solvent enhanced pressure strategy that induces defects (oxygen vacancies) into TiO2 via N doping and reduces its size by using mutual-solvent ethanol and dopant dimethylformamide as pressure-increased reagent of tetrabutyl orthotitanate tetramer is proposed to fabricate N-doped TiO2/C nanocomposites. The induced defects can increase ion storage sites, improve electrical conductivity, and decrease bandgap and ion diffuse energy barrier of TiO2. The size reduction increases contact interfaces between TiO2 and C and shortens ion diffuse distance, thus increasing extra ion storage sites and boosting ion diffusion rate of TiO2. The N-doped TiO2 possesses highly stable crystal structure with a slightly increase of 0.86% in crystal lattice spacing and 3.2% in particle size after fully sodiation. Consequently, as a sodium-ion battery anode, the nanocomposite delivers high capacity and superior rate capability along with ultralong cycling life. This work proposes a novel pressure-induced synthesis strategy that provides unique guidance for designing TiO2-based anode materials with high capacity and excellent fast-charging capability.

Cobalt-vanadium sulfide yolk-shell nanocages from surface etching and ion-exchange of ZIF-67 for ultra-high rate-capability sodium ion battery

Development of high-performance metal sulfides anode materials is a great challenge for sodium-ion batteries (SIBs). In this work, a cobalt-based imidazolate framework (ZIF-67) were firstly synthesized and applied as precursor. After the successive surface etching, ion exchange and sulfidation processes, the final cobalt-vanadium sulfide yolk-shell nanocages were obtained (CoS2/VS4@NC) with VS4 shell and CoS2 yolk encapsulated into nitrogen doped carbon frameworks. This yolk-shell nanocage structure effectively increases the specific surface area and provides enough space for inhibiting the volume change during charge/discharge processes. Besides, the nitrogen doped carbon skeleton greatly improves the ionic conductivity and facilitates ion transport. When used as the anode materials for SIBs, the yolk-shell nanocages of CoS2/VS4@NC electrode exhibits excellent rate capability and stable cycle performance. Notably, it displays a long-term cycling stability with excellent capacity of 417.28 mA h g-1 after 700 cycles at a high current density of 5 A/g. This developed approach here provides a new route for the design and synthesis of various yolk-shell nanocages nanomaterials from enormous MOFs with multitudinous compositions and morphologies and can be extended to the application into other secondary batteries and energy storage fields.

In Situ Transformation of Metal Molybdates into Polymetallic Sulfides with Enriched Edge Sites for High-Performance Supercapacitors

An effective approach to synthesize polycrystalline Ni-Co-Mo sulfide (NiCoMoS) is developed through doping engineering coupled with chemical transformation. The polycrystalline NiCoMoS with enriched active edge sites is designed and fabricated on a Ni foam (NF) via a facile hydrothermal calcination and post-sulfidation approach, where the polycrystalline NiCoMoO₄ precursor is elaborately prepared by doping Co ions into the NiMoO₄ lattice and subsequently in-situ-converted into NiCoMoS with 3D architectures of ordered nanoneedle arrays. Benefiting from the unique 3D structure and synergistic effects of each component, the optimized needle-like NiCoMoS_(2.0) arraying on a NF as a self-standing electrode exhibits superior electrochemical performances with a high specific charge (920.0 C g^-1 at 1.0 A g^-1), excellent rate capability, and good long-term stability. Furthermore, the assembled NiCoMoS//activated carbon hybrid device presents a satisfactory supercapacitor performance, affording an energy density of 35.2 W h kg^-1 at a power density of 800.0 W kg^-1 and competitive long-term stability (83.8% retention at 15 A g^-1 after 10,000 cycles). Such a novel strategy may pave a new route for exploring other polymetallic sulfides with enriched, exposed active edge sites for energy-related applications.

Polyphosphazene-derived P/S/N-doping and carbon-coating of yolk-shelled CoMoO4 nanospheres towards enhanced pseudocapacitive lithium storage

Transition metal oxides as potentialanodes of lithium-ion batteries (LIBs) possess high theoretical capacity but suffer from large volume expansion and poor conductivity. To overcome these drawbacks, we designed and fabricated polyphosphazene-coated yolk-shelled CoMoO₄ nanospheres, in which polyphosphazene with abundant C/P/S/N species was readily converted into carbon shells and provided P/S/N dopants. This resulted in the formation of P/S/N co-doped carbon-coated yolk-shelled CoMoO₄ nanospheres (PSN-C@CoMoO₄). The PSN-C@CoMoO₄ electrode exhibits superior cycle stability of 439.2 mA h g^-1at 1000 mA g^-1after 500 cycles and rate capability of 470.1 mA h g^-1at 2000 mA g^-1. The electrochemical and structural analyses reveal that PSN-C@CoMoO₄ with yolk-shell structure, coated with carbon and doped with heteroatom not only greatly enhances the charge transfer rate and reaction kinetics, but also efficiently buffers the volume variation upon lithiation/delithiation cycling. Importantly, the use of polyphosphazene as coating/doping agent can be a general strategy for developing advanced electrode materials.

Nickel sulfide/nickel phosphide heterostructures anchored on porous carbon nanosheets with rapid electron/ion transport dynamics for sodium-ion half/full batteries

Nickel-based materials have been extensively deemed as promising anodes for sodium-ion batteries (SIBs) owing to their superior capacity. Unfortunately, the rational design of electrodes as well as long-term cycling performance remains a thorny challenge due to the huge irreversible volume change during the charge/discharge process. Herein, the heterostructured ultrafine nickel sulfide/nickel phosphide (NiS/Ni₂P) nanoparticles closely attached to the interconnected porous carbon sheets (NiS/Ni₂P@C) are designed by facile hydrothermal and annealing methods. The NiS/Ni₂P heterostructure promotes ion/electron transport, thus accelerating the electrochemical reaction kinetics benefited from the built-in electric field effect. Moreover, the interconnected porous carbon sheets offer rapid electron migration and excellent electronic conductivity, while releasing the volume variance during Na⁺ intercalation and deintercalation, guaranteeing superior structural stability. As expected, the NiS/Ni₂P@C electrode exhibits a high reversible specific capacity of 344 mAh g^-1 at 0.1 A g^-1 and great rate stability. Significantly, the implementation of NiS/Ni₂P@C//Na₃(VPO₄)₂F₃ SIB full cell configuration exhibits relatively satisfactory cycle performance, which suggests its widely practical application. This research will develop an effective method for constructing heterostructured hybrids for electrochemical energy storage.

Polyvinylpyrrolidone-regulated synthesis of hollow manganese vanadium oxide microspheres as a high-performance anode for lithium-ion batteries

We report the fabrication of well-defined phase-pure Mn₂V₂O₇ hollow microspheres (h-MVO), assembled from the porous plate-like building blocks, via a facile solvothermal method followed by annealing, with the assistance of polyvinylpyrrolidone (PVP) as the structure-regulating agent. The microstructure dependent electrochemical properties of h-MVO as anode materials for lithium ion batteries (LIBs) are investigated, and excellent lithium storage performance is obtained with a reversible capacity of 1707 mAh g^-1 after 700 cycles at 0.5 A g^-1, revealing that the unique hierarchical framework of the h-MVO microspheres with hollow interiors and porous building blocks could not only accelerate the transport of Li⁺ ions and electrolyte, but also efficiently suppress the electrode pulverization upon cycling. More importantly, we demonstrate that PVP can be an effective agent to tune the microstructures, which would be promising for the development of high-performance energy storage devices.

Co-Construction of Solid Solution Phase and Void Space in Yolk-Shell Fe0.4Co0.6S@N-Doped Carbon to Enhance Cycling Capacity and Rate Capability for Aluminum-Ion Batteries

Rechargeable aluminum-ion batteries (AIBs), using low-cost and inherent safety Al metal anodes, are regarded as promising energy storage devices next to lithium-ion batteries. Currently, one of the greatest challenges for AIBs is to explore cathodes suitable for feasible Al³⁺ insertion/extraction with high structure stability. Herein, a facile co-engineering on solid solution phase and cavity structure is developed via Prussian blue analogues by a simple and facile sulfidation strategy. The obtained uniform yolk-shell Fe_0.4Co_0.6S@N-doped carbon nanocages (y-s Fe_0.4Co_0.6S@NC) display a high reversible capacity of 141.3 mA h g^-1 at 500 mA g^-1 after 100 cycles and a good rate capability of 100.9 mA h g^-1 at 1000 mA g^-1. The improved performance can be mainly ascribed to the dual merits of the composite; that is, more negative Al³⁺ formation energy and improved Al³⁺ diffusion kinetics favored by the solid solution phase and Al³⁺ insertion/extraction accommodable space stemmed from the yolk-shell structure. Moreover, the reaction mechanism study discloses that the reaction involves the intercalation of Al³⁺ ions into Fe_0.4Co_0.6S to generate Al_lFe_mCo_nS and elemental Fe and Co.

MoO42--mediated engineering of Na3V2(PO4)3 as advanced cathode materials for sodium-ion batteries

Sodium vanadium phosphate [Na₃V₂(PO₄)₃] with high voltage platform, low cost and environment friendliness has been considered as one of the most promising candidates as cathodes for high-performance sodium-ion batteries. However, the sodium storage property of Na₃V₂(PO₄)₃ is limited because of its low electronic conductivity and poor kinetic performance. Herein, MoO₄^2--doped Na_(3+2x)V₂(PO₄)_(3-x)MoO_4(x) [NVP-MoO₄ (x), x = 0, 0.05, 0.10, 0.15] have been developed and prepared by a feasible solid-state reaction. The optimal NVP-MoO₄ (0.10) delivers a high initial capacity of 108.9 mA h g^-1 and presents an excellent capacity retention of 91.5% at 1 C after 150 cycles. In addition, the NVP-MoO₄ (0.10) shows a good rate capability, delivering a relatively high capacity of 84.2 mA h g^-1 at 50 C. The results of sodium storage measurement and density of states calculation indicate that MoO₄^2- doping can significantly enhance the structural stability, promote the kinetics behavior and boost the electronic conductivity of the materials. In-situ XRD test reveals that the electrochemical reaction of the NVP-MoO₄ (0.10) exhibits a highly reversible phase transition process. This work provides a new insight for the design of advanced cathodes for high-performance sodium-ion batteries by the strategy of unique anion doping.

Metal-organic framework derived vanadium-doped TiO2@carbon nanotablets for high-performance sodium storage

Titanium dioxide (TiO₂) as a potential anode material for sodium-ion batteries (SIBs) suffers from the intrinsic poor electronic conductivity and sluggish ionic diffusivity, thus usually leading to the inferior electrochemical performance. Herein, we demonstrate a facile strategy to enhance the sodium storage performance of TiO₂via vanadium (V) doping, using the pre-synthesized V-doped Ti-based metal-organic framework (MOF, MIL-125) as the precursor, which can be converted into the V-doped TiO₂ with simultaneous carbon hybridization and controlled V-doping amount (denote as V_xTiO₂@C, where × represents the V/Ti molar ratio (R_V/Ti)). V-doping not only affects the morphology of the MIL-125 changing from thick to thin nanotablets, but also greatly enhances the electrochemical performance of the V_xTiO₂@C. When used as an anode for SIBs, the V_0.1TiO₂@C exhibits a much higher reversible capacity of 211 mAh/g than that for the undoped TiO₂@C (only 156 mAh/g) after 150 cycles at 100 mA/g. Even after high-rate long-term cycling, the V_0.1TiO₂@C can still display a capacity of 180 mAh/g with a high capacity retention of 137% at 1000 mA/g after 4500 cycles. Structural/electrochemical measurements reveal that V-doping induces the formation of oxygen vacancies as well as Ti³⁺ species, which efficiently improve the electric conductivity and the ion diffusivity of the electrode. Meanwhile, the thinner V_0.1TiO₂@C nanotablets with porous structure and carbon hybridization could facilitate the ion/electron transfer with shortened diffusion pathways.

Boosting potassium-storage performance via confining highly dispersed molybdenum dioxide nanoparticles within N-doped porous carbon nano-octahedrons

The development of durable and stable metal oxide anodes for potassium ion batteries (PIBs) has been hampered by poor electrochemical performance and ambiguous reaction mechanisms. Herein, we design and fabricate molybdenum dioxide (MoO₂)@N-doped porous carbon (NPC) nano-octahedrons through metal-organic frameworks derived strategy for PIBs with MoO₂ nanoparticles confined within NPC nano-octahedrons. Benefiting from the synergistic effect of nanoparticle level of MoO₂ and N-doped carbon porous nano-octahedrons, the MoO₂@NPC electrode exhibits superior electron/ion transport kinetics, excellent structural integrity, and impressive potassium-ion storage performance with enhanced cyclic stability and high-rate capability. The density functional theory calculations and experiment test proved that MoO₂@NPC has a higher affinity of potassium and higher conductivity than MoO₂ and N-doped carbon electrodes. Kinetics analysis revealed that surface pseudocapacitive contributions are greatly enhanced for MoO₂@NPC nano-octahedrons. In-situ and ex-situ analysis confirmed an intercalation reaction mechanism of MoO₂@NPC for potassium ion storage. Furthermore, the assembled MoO₂@NPC//perylenetetracarboxylic dianhydride (PTCDA) full cell exhibits good cycling stability with 72.6 mAh g^-1 retained at 100 mA g^-1 over 200 cycles. Therefore, this work present here not only evidences an effective and viable structural engineering strategy for enhancing the electrochemical behavior of MoO₂ material in PIBs, but also gives a comprehensive insight of kinetic and mechanism for potassium ion interaction with metal oxide.

Phase structure engineering of MnCo2Ox within electrospun carbon nanofibers towards high-performance lithium-ion batteries

Metal oxides are prospective alternative anode materials to the commercial graphite for lithium ion batteries (LIBs), while their practical application is seriously hampered by their poor conductivities and large volume changes. Herein, we report the controllable synthesis of amorphous/crystalline MnCo₂O_x nanoparticles within porous carbon nanofibers (marked as MCO@CNFs) through a facile electrospinning strategy and subsequent annealing reactions. The phase structures from Co/MnO_X to amorphous MnCo₂O_x and crystalline MnCo₂O_4.5 can be readily tuned by thermal reduction/oxidation under controlled atmosphere and temperature. When examined as anode for LIBs, the optimized MCO@CNFs delivers a high stable capacity of 780.3 mA h g^-1 at 200 mA g^-1 after 250 cycles, which is attributed to the synergistic effect of the distinctive amorphous structure and defective carbon nanofiber matrices. Specifically, the amorphous structure with rich defects offers more reactive sites and multiple pathways for the Li⁺ diffusion, while carbon hybridization sufficiently improves the electrode conductivities as well as buffers the volume changes. More importantly, we demonstrate a convenient synthesis strategy to control the metal-to-oxide structure evolution within carbon matrices, which is of great importance in exploring high-performance electrodes for next generation LIBs.

MOF-Derived CoS2/N-Doped Carbon Composite to Induce Short-Chain Sulfur Molecule Generation for Enhanced Sodium-Sulfur Battery Performance

Dissolution of intermediate sodium polysulfides (Na₂S_x; 4≤x≤8) is a crucial obstacle for the development of room-temperature sodium-sulfur (Na-S) batteries. One promising strategy to avoid this issue is to load short-chain sulfur (S_2-4), which could prohibit the generation of soluble polysulfides during the sodiation process. Herein, unlike in the previous reported cases where short-chain sulfur was stored by confinement within a small-pore-size (≤0.5 nm) carbon host, we report a new strategy to generate short-chain sulfur in larger pores (>0.5 nm) by the synergistic catalytic effect of CoS₂ and appropriate pore size. Based on density functional theory calculations, we predict that CoS₂ can serve as a catalyst to weaken the S-S bond in the S₈ ring structure, facilitating the formation of short-chain sulfur molecules. By experimentally tuning the pore size of the CoS₂-based hosts and comparing their performances as cathodes in Na-S and Li-S batteries, we conclude that such a catalytic effect depends on the proximity of sulfur to CoS₂. This avoids the generation of soluble polysulfides and results in superior electrochemical properties of the composite materials introduced here for Na-S batteries. As a result, the optimized CoS₂/N-doped carbon/S electrode showed excellent electrochemical performance with high reversible specific capacities of 488 mA h g^-1 (962 mA h g_(s)^-1) after 100 cycles (0.1 A g^-1) and 403 mA h g^-1 after 1000 cycles (1 A g^-1) with a superior rate performance (262 mA h g^-1 at 5.0 A g^-1).

Selenizing CoMoO4 nanoparticles within electrospun carbon nanofibers towards enhanced sodium storage performance

Transition metal oxides/selenides as anodes for sodium-ion batteries (SIBs) suffer from the insufficient conductivity and large volumetric expansion, which leads to the poor electrochemical performance. To address these issues, we herein demonstrate a facile selenization method to enhance the sodium storage capability of CoMoO₄ nanoparticles which are encapsulated into the electrospun carbon nanofibers (CMO@carbon for short). The partially and fully selenized CoMoO₄ within carbon nanofibers (denote as CMOS@carbon and CMS@carbon, respectively) can be readily obtained by controlling the annealing temperature (at 400 and 600 °C, correspondingly). When examined as anode materials for SIBs, the CMOS@carbon nanofibers display an outstanding electrochemical performance with a higher reversible capacity of 396 mA h g^-1 after 200 cycles at 0.2 A g^-1 and a high-rate capacity of 365 mA h g^-1 at 2 A g^-1, as compared with the CMO@carbon and CMS@carbon counterparts. The enhanced sodium storage performance of the CMOS@carbon can be owing to the partial selenization of the CoMoO₄ nanoparticles which are rooted into the porous electrospun carbon nanofibers, thus endowing them with superior ionic/electronic charge transfer efficiencies and a cushion against the electrode pulverization during cycling. Moreover, this work proposed a useful strategy to enhance the sodium storage performance of metal oxides via controlled selenization, which is promising for exploiting the advanced anode materials for SIBs.