N-doped carbon encapsulated CoMoO4 nanorods as long-cycle life anode for sodium-ion batteries

Reshaping carbon-coated Mn2Mo3O8 nanotubes and enhanced sodium storage performance

Mn2Mo3O8/C nanotubes are successfully reshaped from micron-sized MnMoO4 blocks using a simple microwave-combined calcination method with dopamine as both scissors and carbon source. The synthesized Mn2Mo3O8/C nanotube (MMOC-2) exhibits enhanced sodium storage performance as anodes for half-cell (217 mA h g-1 with ca. 99% coulombic efficiency after 500 cycles) and full-cell (capacity retention of 75% after 100 cycles), which is attributed to the uniquely reshaped nanostructures with abundant active sites, short ion diffusion path and fast charge transfer.

Stannate-Based Materials as Anodes in Lithium-Ion and Sodium-Ion Batteries: A Review

Binary metal oxide stannate (M₂SnO₄; M = Zn, Mn, Co, etc.) structures, with their high theoretical capacity, superior lithium storage mechanism and suitable operating voltage, as well as their dual suitability for lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs), are strong candidates for next-generation anode materials. However, the capacity deterioration caused by the severe volume expansion problem during the insertion/extraction of lithium or sodium ions during cycling of M₂SnO₄-based anode materials is difficult to avoid, which greatly affects their practical applications. Strategies often employed by researchers to address this problem include nanosizing the material size, designing suitable structures, doping with carbon materials and heteroatoms, metal-organic framework (MOF) derivation and constructing heterostructures. In this paper, the advantages and issues of M₂SnO₄-based materials are analyzed, and the strategies to solve the issues are discussed in order to promote the theoretical work and practical application of M₂SnO₄-based anode materials.

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.

MnMoO4-S nanosheets with rich oxygen vacancies for high-performance supercapacitors

The structure of materials is closely related to their electrochemical properties. MnMoO₄ materials have good stability as supercapacitors but their specific capacitance performance is not excellent. To improve electrochemical performance of MnMoO₄, this study conducts secondary hydrothermal treatment in thiourea solution on MnMoO₄ electrode material grown on nickel foam synthesized by traditional hydrothermal method. A more compact S-doped MnMoO₄ electrode material with more oxygen vacancies and higher specific capacitance was obtained. At the current density of 1 A g^-1, the specific capacitance of the composite material reached 2526.7 F g^-1, which increased by 140.9% compared with that of ordinary MnMoO₄ material. The capacitance retention rate of the composite material was 95.56% after 2000 cycles at 10 A g^-1. An asymmetric supercapacitor was fabricated using S-doped MnMoO₄ as the positive electrode, activated carbon as the negative electrode, and 6 mol L^-1 KOH solution as the electrolyte. The specific capacitance of the assembled supercapacitor was 117.50 F g^-1 at 1 A g^-1, and a high energy density of 47.16 W h kg^-1 at the power density of 849.98 W kg^-1 was recorded. This method greatly improves the specific capacitance of MnMoO₄ through simple processing, which makes it have great application potential.

Metallic Co and crystalline Co-Mo oxides supported on graphite felt for bifunctional electrocatalytic hydrogen evolution and urea oxidation

Oxygen evolution reaction (OER) and urea oxidation reaction (UOR) play important roles in the field of hydrogen energy preparation and pollution treatment. In this work, by merging bimetallic Co-Mo oxides with metallic Co on the graphite felt (GF), we effectively manufacture a 3D bifunctional and highly efficient electrocatalyst (CoMoO@Co/GF) with multi-site functionality for the simultaneous reduction of water and the oxidation of urea in an alkaline medium. The presence of metallic Co causes Co-Mo oxides to evolve from amorphous to crystalline structures. The coupling interface produced between metallic Co and Co-Mo oxides is proven to facilitate electron transport in addition to extensively accessible and highly electroactive Co-Mo oxide nanoflower architecture. The experimental results reveal that the overpotentials for OER and UOR in the CoMoO@Co/GF electrode require only 269 and 115 mV to obtain a current density of 10 mA cm^-2, respectively. Furthermore, with the aid of urea, the overpotential for HER at the current density of 10 mA cm^-2 is lowered to 155 mV. Most notably, the constructed CoMoO@Co/GF-based electrolytic cell only requires a 1.5 V dry battery to achieve effective H₂ evolution and noteworthy stability, outperforming the commercial catalyst-based device and many previous results. The combination of experiments and theoretical calculations further clarifies the active sites in the catalyst. What's more, the pathway of electron transfer in the catalytic process is defined.

General metal-organic framework-derived strategy to synthesize yolk-shell carbon-encapsulated nickelic spheres for sodium-ion batteries

Transition-metal compounds have attracted enormous attention as potential energy storage materials for their high theoretical capacity and energy density. However, the most present transition-metal compounds still suffer from severe capacity decay and limited rate capability due to the lack of robust architectures. Herein, a general metal-organic framework-derived route is reported to fabricate hierarchical carbon-encapsulated yolk-shell nickelic spheres as anode materials for sodium-ion batteries. The nickelic metal-organic framework (Ni-MOF) precursors can be in situ converted into hierarchical carbon-encapsulated Ni₂P (Ni₂P/C), NiS₂ (NiS₂/C) and NiSe₂ (NiSe₂/C) by phosphorization, sulfuration, and selenation reaction, respectively, and maintain their yolk-shell sphere-like morphology. The as-synthesized Ni₂P/C sample can deliver much lower polarization and discharge platform, smaller voltage gap, and faster kinetics in comparison with that of the other two counterparts, and thus achieve higher initial specific capacity (3222.1/1979.3 mAh g^-1) and reversible capacity of 765.4 mAh g^-1 after 110 cycles. This work should provide new insights into the phase and structure engineering of carbon-encapsulated transition-metal compound electrodes via MOFs template for advanced battery systems.

Nanoconfined bimetallic sulfides (CoSn)S heterostructure in carbon microsphere as a high-performance anode for half/full sodium-ion batteries

The development of high-capacity anode materials is crucial for sodium-ion batteries. Alloy-type anode materials have attracted tremendous attention due to their high theoretical capacities. Nonetheless, the realizations of high capacity and remarkable cycling stability are actually hindered by the sluggish reaction kinetics of sodium storage. Here, we report a binary metal sulfides CoS@SnS heterostructure confined in carbon microspheres (denoted as (CoSn)S/C) through a facile hydrothermal reaction combined with annealing treatment. The (CoSn)S/C with micro/nanostructure can shorten ion diffusion length and increase mechanical strength of electrode. Besides, the heterogeneous interface between CoS and SnS can improve the inherent conductivity and favor the rapid transfer of Na⁺. Benefitting from these advantages, (CoSn)S/C composite exhibits a high reversible capacity of 463 mAh g^-1 and superior durability (368 mAh g^-1 at 2 A g^-1 after 1000 cycles). Notably, the assembled Na₃V₂(PO₄)₃//(CoSn)S/C full cell delivers a reversible capacity of 386 mAh g^-1 at 0.2 A g^-1, proving that the (CoSn)S/C is a promising anode material for sodium-ion batteries. The density functional theory (DFT) calculations unveil the mechanism and significance of the constructed CoS@SnS heterostructure for the sodium storage at atomic level. This work provides an important reference for in-depth understanding of reaction kinetics of bimetallic sulfides heterostructure.

Hetero-Element-Doped Molybdenum Oxide Materials for Energy Storage Systems

In order to meet the growing demand for the electronics market, many new materials have been studied to replace traditional electrode materials for energy storage systems. Molybdenum oxide materials are electrode materials with higher theoretical capacity than graphene, which was originally used as anode electrodes for lithium-ion batteries. In subsequent studies, they have a wider application in the field of energy storage, such as being used as cathodes or anodes for other ion batteries (sodium-ion batteries, potassium-ion batteries, etc.), and electrode materials for supercapacitors. However, molybdenum oxide materials have serious volume expansion concerns and irreversible capacity dropping during the cycles. To solve these problems, doping with different elements has become a suitable option, being an effective method that can change the crystal structure of the materials and improve the performances. Therefore, there are many research studies on metal element doping or non-metal doping molybdenum oxides. This paper summarizes the recent research on the application of hetero-element-doped molybdenum oxides in the field of energy storage, and it also provides some brief analysis and insights.

Ultrahigh-energy sodium ion capacitors enabled by the enhanced intercalation pseudocapacitance of self-standing Ti2Nb2O9/CNF anodes

In order to increase the capacity and improve the sluggish Na⁺-reaction kinetics of anodes as sodium ion capacitors (SICs), a Ti₂Nb₂O₉/CNF self-standing film electrode comprised of Ti₂Nb₂O₉ nanosheets and carbon nanofibers has been fabricated via electrospinning HTiNbO₅ nanosheets with PAN and subsequent carbonization treatment. The as-prepared Ti₂Nb₂O₉/CNF film electrode possesses fast Na-ion intercalation kinetics and high conductivity during Na-ion storage, and it displays a high reversible capacity of 324 mA h g^-1 at 0.1 A g^-1. Additionally, it also delivers a superior rate capability of 204 mA h g^-1 at a high current density of 4 A g^-1, as well as an excellent cycling stability of 97% retention after 2000 cycles at 1 A g^-1 in a half-cell test. A prototype Ti₂Nb₂O₉/CNF//AC SIC full device was assembled by employing the presodiated Ti₂Nb₂O₉/CNF anode and AC cathode, and it exhibits an high energy density of 129 W h kg^-1 at a power density of 75 W kg^-1 and a high power density (7560 W kg^-1 with 63 W h kg^-1), a good cycling performance of 85% capacitance retention after 10 000 cycles at 1 A g^-1, suggesting that the Ti₂Nb₂O₉/CNF electrode with excellent performance would be a very promising candidate as the anode for high-performance SICs.

Binary-Metal Mn2SnO4 Nanoparticles and Sn Confined in a Cubic Frame with N-Doped Carbon for Enhanced Lithium and Sodium Storage

Sn-based materials have been popularly researched as anodes for energy storage due to their high theoretical capacity. However, the sluggish reaction kinetics and unsatisfied cycling stability caused by poor conductivity and dramatic volume expansion are still pivotal barriers for the development of Sn-based materials as anodes. In this work, the binary-metal Mn₂SnO₄ nanoparticles and Sn encapsulated in N-doped carbon (Sn@Mn₂SnO₄-NC) were fabricated by multistep reactions and employed as the anode for lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs). The coexistence of binary metals (Sn and Mn) can improve intrinsic conductivity. Simultaneously, hollow architecture along with carbon relieves internal stress and prevents structural collapse. A Sn@Mn₂SnO₄-NC anode delivers an appealing capacity of 1039.5 mAh g^-1 for 100 cycles at 100 mA g^-1 and 823.8 mAh g^-1 for 600 cycles at 1000 mA g^-1 in LIBs. When evaluated as an anode in SIBs, the Sn@Mn₂SnO₄-NC anode tolerates up to 7000 cycles at 2000 mA g^-1 and maintains a capacity of 185.8 mAh g^-1. Quantified kinetic investigations demonstrate the high contribution of pseudocapacitive effects during the cycle process. Furthermore, density functional theory (DFT) calculations further verify that introduction of the second metal (Mn) improves the conductivity of the material, which is favorable for charge transport. This work is expected to provide a feasible preparation strategy for binary-metal materials to enhance the performance of lithium- and sodium-ion batteries.

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.

Hierarchical NiCo2Se4 nanoneedles/nanosheets with N-doped 3D porous graphene architecture as free-standing anode for superior sodium ion batteries

In order to cope with the problem of insufficient lithium metal reserves, sodium ion batteries (SIBs) are proposed and extensively studied for the next-generation batteries. In our work, hierarchical NiCo₂Se₄ nanoneedles/nanosheets are deposited on the skeleton of N-doped three dimensional porous graphene (NPG) by a convenient solvothermal method and subsequent gas-phase selenization process. Compared with NiCo₂Se₄ powder, the optimized NiCo₂Se₄/N-doped porous graphene composite (denoted as NCS@NPG) as self-supporting anode exhibits the excellent electrode activity for SIBs, with a specific capacity of 500 mAh/g and 257 mAh/g at a current density of 0.2 A/g and 6.4 A/g, respectively. The high specific capacity as well as rate capacity can be attributed to the three-dimensional graphene skeleton with high electrical conductivity and pore structure, which provides convenient ion and electron transmission channels.