2D hierarchical yolk-shell heterostructures as advanced host-interlayer integrated electrode for enhanced Li-S batteries

Lithium sulfur(Li-S)batteries hold great promising for high-energy-density batteries,but appear rapid capacity fading due to the lack of overall and elaborated design of both sulfur host and interlayer.Herein,we developed a novel two-dimensional(2D)hierarchical yolk-shell heterostructure,constructed by a graphene yolk,2D void and outer shell of vertically aligned carbon-mediated MoS2 nanosheets(G@void@MoS2/C),as advanced host-interlayer integrated electrode for Li-S batteries.Notably,the 2D void,with a typical thickness of^80 nm,provided suitable space for loading and confining nano sulfur,and vertically aligned ultrathin MoS2 nanosheets guaranteed enriched catalytically active sites to effectively promote the transition of soluble polysulfides.The conductive graphene yolk and carbon mediated shell sufficiently accelerated electron transport.Therefore,the integrated electrode of G@void@MoS2/C not only exceptionally confined the sulfur/polysulfides in 2D yolk-shell heterostructures,but also achieved catalytic transition of the residual polysulfides dissolved in electrolyte to solid Li2S2/Li2S,both of which synergistically achieved an extremely low capacity fading rate of 0.05%per cycle over 1000 times at 2C,outperforming most reported Mo based cathodes and interlayers for Li-S batteries.2D hierarchical yolkshell heterostructures developed here may shed new insight on elaborated design of integrated electrodes for Li-S batteries.

A Review: Enhanced Anodes of Li/Na-Ion Batteries Based on Yolk–Shell Structured Nanomaterials

Lithium-ion batteries(LIBs) and sodium-ion batteries(SIBs) have received much attention in energy storage system. In particular, among the great efforts on enhancing the performance of LIBs and SIBs, yolk–shell(YS) structured materials have emerged as a promising strategy toward improving lithium and sodium storage. YS structures possess unique interior void space, large surface area and short diffusion distance, which can solve the problems of volume expansion and aggregation of anode materials, thus enhancing the performance of LIBs and SIBs. In this review, we present a brief overview of recent advances in the novel YS structures of spheres, polyhedrons and rods with controllable morphology and compositions. Enhanced electrochemical performance of LIBs and SIBs based on these novel YS structured anode materials was discussed in detail.

Hierarchical Carbon Microtube@Nanotube Core-Shell Structure for High-Performance Oxygen Electrocatalysis and Zn-Air Battery

Zinc-air batteries(ZABs)hold tremendous promise for clean and efficient energy storage with the merits of high theoretical energy density and environmental friendliness.However,the performance of practical ZABs is still unsatisfactory because of the inevitably decreased activity of electrocatalysts when assembly into a thick electrode with high mass loading.Herein,we report a hierarchical electrocatalyst based on carbon microtube@nanotube core-shell nanostructure(CMT@CNT),which demonstrates superior electrocatalytic activity for oxygen reduction reaction and oxygen evolution reaction with a small potential gap of 0.678 V.Remarkably,when being employed as air-cathode in ZAB,the CMT@CNT presents an excellent performance with a high power density(160.6 mW cm^−2),specific capacity(781.7 mAhgZn^−1)as well as long cycle stability(117 h,351 cycles).Moreover,the ZAB performance of CMT@CNT is maintained well even under high mass loading(3 mg cm−2,three times as much as traditional usage),which could afford high power density and energy density for advanced electronic equipment.We believe that this work is promising for the rational design of hierarchical structured electrocatalysts for advanced metal-air batteries.

Engineering core–shell Co_(9)S_(8)/Co nanoparticles on reduced graphene oxide: Efficient bifunctional Mott–Schottky electrocatalysts in neutral rechargeable Zn–Air batteries

It is significant for the rational construction of the high–efficient bifunctional electrocatalysts for in–depth understandings of how to improve the electron transfer and ion/oxygen transport in catalyzing oxygen reduction reaction and oxygen evolution reaction(ORR and OER),but still full of vital challenges.Herein,we synthesize the novel“three–in–one”catalyst that engineers core–shell Mott–Schottky Co_(9)S_(8)/Co heterostructure on the defective reduced graphene oxide(Co_(9)S_(8)/Co–rGO).The Co_(9)S_(8)/Co–rGO catalyst exhibits abundant Mott–Schottky heterogeneous–interfaces,the well–defined core–shell nanostructure as well as the defective carbon architecture,which provide the multiple guarantees for enhancing the electron transfer and ion/oxygen transport,thus boosting the catalytic ORR and OER activities in neutral electrolyte.As expected,the integrated core–shell Mott–Schottky Co_(9)S_(8)/Co–rGO catalyst delivers the most robust and efficient rechargeable ZABs performance in neutral solution electrolytes accompanied with a power density of 59.5 mW cm^(-2) and superior cycling stability at 5 mA cm^(-2) over 200 h.This work not only emphasizes the rational designing of the high–efficient bifunctional oxygen catalysts from the fundamental understanding of accelerating the electron transfer and ion/oxygen transport,but also sheds light on the practical application prospects in more friendly environmentally neutral rechargeable ZABs.

Co/Ni dual-metal embedded in heteroatom doped porous carbon core-shell bifunctional electrocatalyst for rechargeable Zn-air batteries

Rational construction of highly efficient and cheap bifunctional electrocatalysts to boost both oxygen evolution reaction(OER)and oxygen reduction reaction(ORR)is extremely essential for the wide application of rechargeable metal-air battery.In this work,we design a core-shell structural catalyst of CoNi dual-metal embedded in nitrogen doped porous carbon(NPC,CoNi@NPC),which is developed via the pyrolysis of CoNiMOFs,assisting by mesoporous SiO_(2) to effectively inhibit the aggregation of metal sites.Consequently,the asprepared CoNi@NPC manifests good ORR activity with half-wave potential up to 0.77 V.Specifically,the CoNi@NPC gives a very low OER over-potential of merely 101 mV in 6 M KOH along with high stability,outperforming the commercial Pt/C-RuO_(2).Moreover,the home-made zinc air battery with CoNi@NPC air cathode demonstrates excellent stability over long-term charging–discharging test,and delivers the maximum power density of 224 mW cm^(-2).The enhanced high performance of CoNi@NPC bifunctional catalyst for both ORR and OER can be ascribed to its unique core-shell structure and strong synergistic effect between the dual-bimetal active sites and the heteroatom doped carbon.This work opens a new avenue for the rational design of nonprecious metal bifunctional catalysts for rechargeable metal-air battery.

Metal–Organic Frameworks-Derived Porous Yolk–Shell MoP/Cu_(3)P@carbon Microcages as High-Performance Anodes for Sodium-Ion Batteries

Transitional metal phosphides(TMPs)anode materials usually have large volume change and weak diffusion kinetics,leading to poor cycle stability.Combining TMPs with conductive carbon matrix has been widely used to boost sodium storage.However,it still needs to make efforts in the rational and facile design of nano/micro-structural TMPs/carbon hybrid anode material.Herein,a MOFs-derived strategy is developed to synthesize porous yolk–shell Mo P/Cu_(3)P@carbon microcages(Mo P/Cu_(3)P@C)through in situ and confined phosphidation reaction as a high-performance sodium-ion batteries anode.This yolk–shell structure can offer adequate internal space to buffer the large volume expansion,shorten diffusion distance,and create more active sites of Na+.Especially,the Cu nanoparticles generated from Cu_(3)P have remarkable electronic conductivity of 5.73107S m-1(the second most conductive metal)to benefit transporting electrons.And the introduction of Mo(Mo P has high theoretical capacity of 633 mA h g^(-1))can enhance the reversible capacity of the whole anode material.Therefore,these porous yolk–shell Mo P/Cu_(3)P@carbon microcages possess excellent reversible capacity of 307.8 mA h g^(-1)at 1.0 A g^(-1)and extraordinary cycle stability of 132.1 m A h g^(-1)at 5.0 A g^(-1)even after 6000 cycles.

Microwave-Assisted Synthesis of NiCo_2O_4 Double-Shelled Hollow Spheres for High-Performance Sodium Ion Batteries

The ternary transitional metal oxide NiCo_2O_4 is a promising anode material for sodium ion batteries due to its high theoretical capacity and superior electrical conductivity. However, its sodium storage capability is severely limited by the sluggish sodiation/desodiation reaction kinetics. Herein, NiCo_2O_4 double-shelled hollow spheres were synthesized via a microwave-assisted, fast solvothermal synthetic procedure in a mixture of isopropanol and glycerol, followed by annealing. Isopropanol played a vital role in the precipitation of nickel and cobalt,and the shrinkage of the glycerol quasi-emulsion under heat treatment was responsible for the formation of the double-shelled nanostructure. The as-synthesized productwas tested as an anode material in a sodium ion battery,was found to exhibit a high reversible specific capacity of 511 m Ahg^(-1) at 100 m Ag^(-1), and deliver high capacity retention after 100 cycles.

Nb_2O_5-carbon core-shell nanocomposite as anode material for lithium ion battery

Nb2O5-carbon nanocomposite is synthesized through a facile one-step hydrothermal reaction from sucrose as the carbon source, and stuclled as an anode material for high-performance lithium ion battery. The structural characterizations reveal that the nanocomposite possesses a core-shell structure with a thin layer of carbon shell homogeneously coated on the Nb2O5 nanocrystals. Such a unique structure enables the composite electrode with a long cycle life by preventing the Nb2O5 from volume change and pulverization during the charge-discharge process. In addition, the carbon shell efficiently improves the rate capability. Even at a current density of 500 mA.g-1, the composite electrode still exhibits a specific capacity of ~100 mAh.g-1. These results suggest the possibility to utilize the Nb2O5-carbon core-shell composite as a high performance anode material in the practical application of lithium ion battery.

Core-shell meso/microporous carbon host for sulfur loading toward applications in lithium-sulfur batteries

Lithium-sulfur（Li-S） batteries belong to one of the promising technologies for high-energy-density rechargeable batteries.However,sulfur cathodes suffer from inherent problems of its poor electronic conductivity and the shuttling of highly dissoluble lithium polysulfides generated during the cycles.Loading sulfur into porous carbons has been proved to be an effective approach to alleviate these issues.Mesoporous and microporous carbons have been widely used for sulfur accommodation,but mesoporous carbons have poor sulfur confinement,whereas microporous carbons are impeded by low sulfur loading rates.Here,a core-shell carbon,combining both the merits of mesoporous carbon with large pore volume and microporous carbon with effective sulfur confinement,was prepared by coating the mesoporous CMK-3 with a microporous carbon（MPC） shell and served as the carbon host（CMK-3 @MPC） to accommodate sulfur.After sulfur infusion,the as-obtained S/（CMK-3@MPC） cathode delivered a high initial capacity of up to 1422 mAh·g-1 and sustained 654 mAh·g-1 reversible specific capacity after 36 cycles at 0.1 C.The good performance is ascribed to the unique core-shell structure of the CMK-3@MPC matrix,in which sulfur can be effectively confined within the meso/microporous carbon host,thus achieving simultaneously high electrochemical utilization.

Core@shell sulfur@polypyrrole nanoparticles sandwiched in graphene sheets as cathode for lithium–sulfur batteries

A nano sulfur-based composite cathode material featured by uniform core@shell-structured sulfur@polypyrrole nanoparticles sandwiched in three-dimensional graphene sheets conductive network（S@PPy/GS） is fabricated via a facile solution-based method. The S@PPy nanoparticles are synthesized by in situ chemical oxidative polymerization of pyrrole on the surface of sulfur particles,and then graphene sheets are covered outside the S@PPy nanoparticles,forming a three-dimensional conductive network. When evaluating the electrochemical performance of S@PPy/GS in a lithium–sulfur battery,it delivers large discharge capacity,excellent cycle stability,and good rate capability. The initial discharge capacity is up to 1040 m Ah/g at 0.1 C,the capacity can remain 537.8 m Ah/g at 0.2 C after 200 cycles,even at a higher rate of 1 C,the specific capacity still reaches 566.5 m Ah/g. The good electrochemical performance is attributed to the unique structure of S@PPy/GS,which can not only provide an excellent transport of lithium and electron ions within the electrodes,but also retard the shuttle effect of soluble lithium polysulfides effectively,thus plays a positive role in building better lithium-sulfur batteries.

Core-shell structured 1,4-benzoquinone@TiO_2 cathode for lithium batteries

Organic carbonyl compounds are considered as promising candidates for lithium batteries due to theirhigh capacity and environmental friendliness, However, they suffer from serious dissolution in the elec-trolyte, leading to fast capacity decay. Here we report core-shell structured 1,4-benzoquinone@titaniumdioxide （BQ@TiO2） composite as cathode for lithium batteries. The composite cathode can deliver a highdischarge capacity of 441.2 mA h/g at 50 mA/g and a high capacity retention of 80.7% after 100 cycles. Thegood cycling performance of BQ@TiO2 composite can be attributed to the suppressed dissolution of BQ,which results from the physical confinement effect of Ti02 shell and the strong interactions between BQand Ti02. Moreover, the combination of ex situ infrared spectra and density functional theory calculationsreveals that the active redox sites of BQ are carbonyl groups. This work provides an alternative way tomitigate the dissolution of small carbonyl compounds and thus enhance their cycling stability.

Facile Fabrication of Fe3O4@TiO2@C Yolk–Shell Spheres as Anode Material for Lithium Ion Batteries

Transition metal oxides have been actively exploited for application in lithium ion batteries due to their facile synthesis,high specific capacity,and environmental-friendly.In this paper,Fe3O4@TiO2@C yolk-shell(Y-S)spheres,used as anode material for lithium ion batteries,were successfully fabricated by Stober method.XRD patterns reveal that Fe3O4@TiO2@C Y-S spheres possess a good crystallinity.But the diffraction peaks’intensity of Fe3O4 crystals in the composites is much weaker than that of bare Fe3O4 spheres,indicating that the outer anatase TiO2@C layer can cover up the diffraction peaks of inner Fe3O4 spheres.The yolk-shell structure of Fe3O4@TiO2@C spheres is further characterized by TEM,HAADFSTEM,and EDS mapping.The yolk-shell structure is good for improving the cycling stability of the inner Fe3O4 spheres during lithium ions insertion-extraction processes.When tested at 200 mA/g,the Fe3O4@TiO2@C Y-S spheres can provide a stable discharge capacity of 450 mAh/g over 100 cycles,which is much better than that of bare Fe3O4 spheres and TiO2@C spheres.Furthermore,cyclic voltammetry curves show that the composites have a good cycling stability compared to bare Fe3O4 spheres.

Porous core–shell CoMn_2O_4 microspheres as anode of lithium ion battery with excellent performances and their conversion reaction mechanism investigated by XAFS

Porous core-shell CoMn204 microspheres of ca. 3-5μm in diameter were synthesized and served as an-ode of lithium ion battery. Results demonstrate that the as-synthesized CoMn204 materials exhibit excel-lent electrochemical properties. The CoMn204 anode can deliver a large capacity of 1070 mAh g-1 in thefirst discharge, a reversible capacity of 500 mAh g^-1 after 100 cycles with a coulombic efficiency of 98.5% at a charge-discharge current density of 200 mA g^-l, and a specific capacity of 385 mAh g^-1 at a muchhigher charge-discharge current density of 1600mA g^-1. Synchrotron X-ray absorption fine structure（XAFS） techniques were applied to investigate the conversion reaction mechanism of the CoMn204 anode.The X-ray absorption near edge structure （XANES） spectra revealed that, in the first discharge-charge cy-cle, Co and Mn in CoMn204 were reduced to metallic Co and Mn when the electrode was discharged to0.01 V, while they were oxidized respectively to CoO and MnO when the electrode was charged to 3.0V.Experiments of both XANE5 and extended X-ray absorption fine structure （EXAFS） revealed that neithervalence evolution nor phase transition of the porous core-shell CoMn204 microspheres could happen inthe discharge plateau from 0.8 to 0.6V, which demonstrates the formation of solid electrolyte interface（SEI） on the anode.

Facile preparation of core-shell Si@Li4Ti5O12 nanocomposite as large-capacity lithium-ion battery anode

As a promising alternative anode material,silicon(Si)presents a larger capacity than the commercial anode to achieve large capacity lithium-ion batteries.However,the application of pure Si as anode is hampered by limitations such as volume expansion,low conductivity and unstable solid electrolyte interphase.To break through these limitations,the core-shell Si@Li4Ti5O12nanocomposite,which was prepared via in-situ self-assembly reaction and decompressive boiling fast concentration method,was proposed in this work.This anode combines the advantages of nano-sized Si particle and pure Li4Ti5O12(LTO)coating layer,improving the performance of the lithium-ion batteries.The Si@Li4Ti5O12 anode displays a high initial discharge/charge specific capacity of 1756/1383 m Ahg^-1 at 500 mAg^-1(representing high initial coulombic efficiency of 78.8%),a large rate capability(specific capacity of 620 mAhg^-1 at4000 mAg^-1),an outstanding cycling stability(reversible specific capacity of 883 mAhg^-1 after 150 cycles)and a low volume expansion rate(only 3.3% after 150 cycles).Moreover,the synthesis process shows the merits of efficiency,simplicity,and economy,providing a reliable method to fabricate large capacity Si@Li4Ti5O12nanocomposite anode materials for practical lithium-ion batteries.

Hard carbons derived from pine nut shells as anode materials for Na-ion batteries

Hard carbons as promising anode materials for Na-ion batteries(NIBs) have captured extensive attention because of their low operation voltage, easy synthesis process, and competitive specific capacity. However, there are still several disadvantages, such as high cost and low initial coulombic efficiency, which limit their large-scale commercial applications.Herein, pine nut shells(PNSs), a low-cost biomass waste, are used as precursors to prepare hard carbon materials. Via a series of washing and heat treatment procedures, a pine nut shell hard carbon(PNSHC)-1400 sample has been obtained and delivers a reversible capacity of around 300 mAh/g, a high initial coulombic efficiency of 84%, and good cycling performance. These excellent Na storage properties indicate that PNSHC is one of the most promising candidates of hard carbon anodes for NIBs.

Lithium Storage Performance of Hollow and Core/Shell TiO_2 Microspheres Containing Carbon

TiO2 microspheres containing carbon have been synthesized viaa one-pot hydrothermal process using CTAB as the mesoporous template and nanoparticle stabilizer and Ti(SO4)2and sucrose as titanium and carbon precursors,respectively. Through well designed calcinations, Ti O2 microspheres with various amounts of carbon-residue,such as core/shell C@TiO2, hollow neat H–TiO2, and hollow C/TiO2 composites, are obtained. When these microspheres are used as anode materials for lithium ion batteries, the lithium storage performance is significantly influenced by the structure and carbon-residue. With a thin shell of TiO2 nanoparticles and carbon-residue, the capacity of hollow C/TiO2 composites maintains at 143.3 m A·h·g-1at 0.5 C(83.5 m A·g-1) after 100 cycles.Moreover, after high rate charge/discharge cycles from 0.2 C to 20 C and back to 0.2 C again, the reversible capacity recovers atas high as 195.1 m A·h·g-1with respect to its initial value of 205.0 m A·h·g-1. The results of cycle voltammograms and electrochemical impedance spectroscopy further reveal that Li+insertion/extraction processes are reversible, and the diffusion coefficient of Li+in the hollow C/TiO2 composites is much higher than those of others, because the hollow structure can act as the ion-buffering reservoir and facilitate Li+transfer from both sides of the shell, and the carbon-residue in the shell improves the conductivity as well.

High performance columnar-like Fe2O3@carbon composite anode via yolk@shell structural design

Conversion-type reaction anode materials with high specific capacity are attractive candidates to improve lithium ion batteries(LIBs), yet the rapid capacity fading and poor rate capability caused by drastic volume change and low electronic conductivity greatly hinder their practical applications. To circumvent these issues, the successful design of yolk@shell Fe2 O3@C hybrid composed of a columnar-like Fe2O3 core within a hollow cavity completely surrounded by a thin, self-supported carbon(C) shell is presented as an anode for high-performance LIBs. This yolk@shell structure allows each Fe2O3 core to swell upon lithiation without deforming the carbon shell. This preserves the structural and electrical integrity against pulverization, as revealed by in situ transmission electron microscopy(TEM) measurement. Benefiting from these structural advantages, the resulting electrode exhibits a high reversible capacity(1013 m Ah g-1 after80 cycles at 0.2 A g-1), outstanding rate capability(710 m Ah g-1 at 8 A g-1) and superior cycling stability(800 m Ah g-1 after 300 cycles at 4 A g-1). A Li-ion full cell using prelithiated yolk@shell Fe2 O3@C hybrid as the anode and commercial Li CoO2(LCO) as the cathode demonstrates impressive cycling stability with a capacity retention of 84.5% after 100 cycles at 1 C rate, holding great promise for future practical applications.

Bio-Derived Hierarchical Multicore–Shell Fe2N-Nanoparticle-Impregnated N-Doped Carbon Nanofiber Bundles:A Host Material for Lithium-/Potassium-Ion Storage

Despite the significant progress in the fabrication of advanced electrode materials,complex control strategies and tedious processing are often involved for most targeted materials to tailor their compositions,morphologies,and chemistries.Inspired by the unique geometric structures of natural biomacromolecules together with their high affinities for metal species,we propose the use of skin collagen fibers for the template crafting of a novel multicore-shell Fe2N-carbon framework anode configuration,composed of hierarchical N-doped carbon nanofiber bundles firmly embedded with Fe2N nanoparticles(Fe2N@N-CFBs).In the resultant heterostructure,the Fe2N nanoparticles firmly confined inside the carbon shells are spatially isolated but electronically well connected by the long-range carbon nanofiber framework.This not only provides direct and continuous conductive pathways to facilitate electron/ion transport,but also helps cushion the volume expansion of the encapsulated Fe2N to preserve the electrode microstructure.Considering its unique structural characteristics,Fe2N@N-CFBs as an advanced anode material exhibits remarkable electrochemical performances for lithium-and potassium-ion batteries.Moreover,this bio-derived structural strategy can pave the way for novel low-cost and high-efficiency syntheses of metal-nitride/carbon nanofiber heterostructures for potential applications in energy-related fields and beyond.

Two-in-one shell configuration for bimetal selenides toward fast sodium storage within broadened voltage windows

The shell structure design has been recognized as a highly efficient strategy to buffer the severe volume expansion and consecutive pulverization of conversion-type anodes.Nevertheless,construction of a functional shell with a stabilized structure that meets the demands of both high electronic conductivity and feasible pathways for Na^(+)ions has been a challenge so far.Herein,we design a two-in-one shell configuration for bimetal selenides to achieve fast sodium storage within broadened voltage windows.The hybridized shell,which benefits from the combination of titanium dioxide quantum dots and amorphous carbon,can not only effectively buffer the strain and maintain structural integrity but also allow facile and reversible transport of electrons and Na^(+)uptake for electrode materials during sodiation/desodiation processes,resulting in increased reaction kinetics and diffusion of sodium ions,conferring many benefits to the functionality of conversion-type electrode materials.As a representative material,Ni-CoSe_(2) with such structural engineering shows a reversible capacity of 515 mAh g^(−1)at 0.1 A g^(−1)and a stable capacity of 416 mAh g^(−1)even at 6.4 A g^(−1);more than 80%of the capacity at 0.1 A g^(−1)could be preserved,so that this strategy holds great promise for designing fast-charging conversion-type anodes in the future.

A polypyrrole-coated acetylene black/sulfur composite cathode material for lithium–sulfur batteries

Lithium–sulfur batteries are promising next-generation energy storage devices beyond conventional lithium ion batteries. However, it suffers from rapid capacity fading and poor cyclic stability. Here we report a facile in situ sulfur deposition and chemical oxidative polymerization method for preparing acetylene black/sulfur@polypyrrole（AB/S@PPy） composite as a cathode material for lithium–sulfur batteries. It is demonstrated that PPy is covered uniformly onto the surface of the AB/S composite forming a core–shell structure. In the structure, AB in the matrix and PPy on the surface acts as a combined conductive framework to provide ions and electrons transport pathways, and to inhibit the dissolution or diffusion of polysulfide into the electrolyte. The as-designed AB/S@PPy composite exhibits excellent rate capability and cyclic stability. The initial discharge specific capacity is as high as 1179.4 m Ah/g, and remains at769.3 m Ah/g after 80 cycles at 0.2 C. Even at a high rate（0.5 C）, a maximum discharge capacity of 811.1 m Ah/g is still achieved for the AB/S@PPy composite after activation, and the capacity retention is over62.5% after 200 cycles.