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The microflower-like composites, constructed by two-dimensional core/shell FeSe/carbon nanosheet, are synthesized based on carbon-confined oriented-attachment mechanism. This unique structure can effectively enhance sodium-ion diffusion kinetics, accelerate the electron transfer and inhibit the volume expansion of FeSe kernel. When used as anode materials of sodium ion batteries (SIBs), the composites can deliver a high capacity of 460.2 mA h g−1 with ultrahigh initial Coulombic efficiency (ICE) of 99.8%. Moreover, the composites exhibit excellent rate-performance and super-long cycling stability. The capacity can be as high as 343.8 and 341.3 mA h g−1 at 5 and 10 A g−1. At ultrahigh current density of 30 A g−1, the composites can still deliver a capacity of 183.8 mA h g−1 and maintain a value of ca. 100 mA h g−1 after 10000 cycles. Interestingly, the composites can be directly assembled with Na3V2(PO4)3 into full battery without presodiation. The full battery delivers a capacity of 214.6 mA h g−1 after 140 cycles with a retention of 99.9% at 1 A g−1, showing superior rate and cyclic performances. Accordingly, this work is of great significant for design of non-presodiate anode materials to promote the practical applications of SIBs.

Hard carbons have shown promising application potential as anode materials for sodium-ion batteries (SIBs), but adjusting the texture of hard carbons to manipulate their electrochemical behaviors remains a great challenge. In this work, a Cu-activation strategy is developed to control the defects of hard carbon nanofibers to achieve slope-reigned Na-ion storage behaviors. This method can effectively create defect-rich carbon texture by employing a small amount of Cu(NO

Electrospun hard carbon nanofibers (CNFs) have been considered as promising anode materials for sodium-ion batteries (SIBs). The beads-on-string nanofibers have been frequently appeared as a universal phenomenon during electrospinning, but the influence of beads-on-string on Na-ion storage have never been investigated. Here, in order to clarify this issue, the electrospun polyimide-based CNFs with/without beads-on-string are designed by controlling the concentration of electrospinning solution, and their Na-ion storage properties are investigated as free-standing electrode and slurry-coating electrode, respectively. Generally, the discharge/charge curves of hard carbons consist of slope- and plateau-regions. When used as free-standing electrode, the beads-on-string can depress Na-ion storage behavior of plateau-region, but do not influence storage behavior of slope-region, and, interestingly, the plateau capacity increases with the decreasing of the beads-on-string, while the slope capacity is unaffected by the beads-on-string. When used as slurry-coating electrodes, the beads-on-string can hardly influence Na-ion storage behaviors of slope- and plateau-regions, and CNFs with/without beads-on-string display nearly similar slope and plateau capacities and rate-performance. Moreover, benefiting from the special micro-nanostructure, beads-on-string CNFs exhibit better high rate cycling stability than ones without beads as slurry-coating electrode. Therefore, these interesting phenomena will provide valuable inspiration for designing advanced electrospun CNFs electrode for SIBs.

The design of carbon anode materials with both high gravimetric and volumetric performances is one of the main limitations in the real applications of sodium-ion batteries (SIBs). Herein, we develop novel micro/nanostructure carbon microspheres (CMSs) assembled of onion-like carbons via an in situ pyrolysis approach. When used as anode materials for SIBs, the CMSs electrode possesses a high pressing density of 1.36 g cm−3. The CMSs exhibit a high reversible gravimetric capacity of 275 mAh g−1 and a reversible volumetric capacity of 374 mAh cm−3 at 100 mA g−1, which can be comparable to the volumetric capacity of graphite for lithium-ion batteries. At high current densities of 1, 2 and 5 A g−1, the reversible gravimetric capacities can still be maintained at 195, 165 and 115 mAh g−1 and the volumetric capacities can reach up to 265, 224 and 156 mAh cm−3. Such excellent electrochemical performances should be attributed to the special spherical micro/nanostructure of CMSs, which can not only guarantee a high density, but also provide void spaces to alleviate the volume variation and enhance Na ions diffusion kinetics. Accordingly, this study proposes a new strategy for the design of carbon materials with both high gravimetric and volumetric performances.

A flake-like carbon coated Mn2SnO4 nanomaterial was synthesized via a two-step carbonization method using Mn-based metal-organic frameworks (Mn-MOFs) as precursor. The composite demonstrates a two-dimensional (2D) micro/nano-scale combined configuration with Mn2SnO4 nanoparticles dispersed uniformly in flake-like porous carbon matrix. The unique 2D and porous morphology is favorable for lithium ions (Li-ions) storage due to higher surface for electrode/electrolyte interaction and reducing diffusion length of Li-ions. This material gives a high specific capacity of 986 mA h g−1 with 90.1% capacity retention after 100 cycles at 100 mA g−1 and a capacity of 428 mA h g−1 even at a high current density of 2 A g−1. Moreover, the performance of the carbon coated Mn2SnO4 was compared with carbon coated MnO/SnO2 which has the same element composition with Mn2SnO4. It is believed that the “synergistic effect” in Mn2SnO4 helps to improve the reversibility of the lithium storage reactions, resulting in both higher capacity and better stability of carbon coated Mn2SnO4.

Metal-organic-frameworks (MOFs) have recently been an emerging self-templated precursor to derive various nanomaterials for Na-ion storage. However, synthesis of MOF-derived two-dimensional nanomaterials has been rarely explored. Here, Ni(NO3)2/hexamine coordination-frameworks are employed as a precursor to prepare NiTe2/nitrogen-doped graphitic carbon nanosheets (NiTe2@NCNs) via pyrolysis and subsequent tellurization. Layered NiTe2 nanoparticles are uniformly anchored on the carbon nanosheets. As anode materials, NiTe2@NCNs exhibit ultrafast and long-life Na-ion storage performance. NiTe2@NCNs can deliver a reversible capacity of 289.5 mA h g−1 at 0.1 A g−1 and exhibit a negligible capacity fading with the increasing of current density from 0.1 to 10 A g−1. Even at 10 and 20 (ca. 70 C) A g−1, reversible capacities remain at 267.7 and 189.0 mA h g−1, respectively, and cyclic life can be up to 5000 cycles. Besides, NiTe2@NCNs exhibit minimal increasing of polarization from 0.1 to 5 A g−1. The superior electrochemical performance of NiTe2@NCNs should be attributed to the two-dimensional structure, N-doped graphitic carbon matrix and lager interlayer spacing of NiTe2, which contribute to enhance kinetics of electron and Na-ion transfer so as to result in a capacitive-controlled Na-ion storage process. Therefore, NiTe2@NCNs have a promising potential as anode materials for practical Na-ion storage applications.

Exploring new electrode materials with both high gravimetric and volumetric Na-ion storage performances is urgently desired but still faces significant challenges. In this work, a composite of layered Bi2Te3 nanoplates/graphene, which is denoted as Bi2Te3/G, was synthesized by a one-pot solvothermal method. Here, Bi2Te3 nanoplates with a large interlayer spacing of 1.02 nm were uniformly anchored on graphene with strong interfacial interaction. The obtained Bi2Te3/G composite exhibited high reversible gravimetric capacity of 416 mA h g−1 at 0.1 A g−1 and an excellent rate performance. Even at 5.0 A g−1, Bi2Te3/G still delivered a gravimetric capacity as high as 203 mA h g−1. Due to its high tap density of 1.56 g cm−3, Bi2Te3/G also delivered impressive volumetric capacities. The reversible volumetric capacity was as high as 648.9 mA h cm−3 at 0.1 A g−1. At 5 A g−1, Bi2Te3/G still retained volumetric capacity of 316.7 mA h cm−3. The excellent gravimetric and volumetric Na-ion storage performances of the Bi2Te3/G composite should be ascribed to the quintuple layer sandwich structure of Bi2Te3, the synergistic effect between Bi2Te3 and graphene, and its higher density. Accordingly, the Bi2Te3/G composite is expected to exhibit great application prospects as a promising anode candidate for smart sodium-ion batteries.

Transition metal fluorides (TMFs) have attracted much attention as electrode materials for sodium-ion batteries (SIBs) due to their abundance, low cost, and high specific capacity. However, intrinsic low electronic conductivity and high resistance to Na ion diffusion impede the applications of TMFs in SIBs. Herein, diperovskite-type (NH4)3FeF6/graphene nanosheet ((NH4)3FeF6/GNS) composites were successfully synthesized using a facile co-pyrolysis approach from ferric acetylacetonate (Fe(acac)3) and ammonium fluoride (NH4F) with the addition of GNS. The pure diperovskite phase of (NH4)3FeF6 was generated at reaction temperatures ranging from 150–300 °C using a high NH4F/Fe(acac)3 molar ratio. The stable diperovskite structure, bearing an open three-dimensional framework, was favourable for the fast storage of large-sized Na ions. Therefore, the (NH4)3FeF6/GNS composites exhibited a high capacity of 487.8 mA h g−1 at 0.1 A g−1 and excellent rate performances as anode materials for SIBs. The specific capacities of the (NH4)3FeF6/GNS composites were up to 257.8, 219.4, and 180.6 mA h g−1 at 2.0, 5.0, and 10.0 A g−1, respectively. Even at 20.0 A g−1 (approx. 58C), the (NH4)3FeF6/GNS composites still delivered a specific capacity of 149.5 mA h g−1 and exhibited a long lifespan up to 3000 cycles. Therefore, the (NH4)3FeF6/GNS composites are potential anode materials for SIBs.

The development of anode materials for both sodium-ion batteries (SIBs) and potassium-ion batteries (KIBs) with promising performance is of fundamental and technological significance. In this work, N-rich hollow carbon-onion-constructed nanosheets (HCONs) are developed from Co–hexamine coordination frameworks. Impressively, as anode materials for SIBs, the HCONs demonstrate an ultrastable Na-ion storage capability of 151 mA h g−1 at 5 A g−1 even after 10 000 cycles, with a capacity retention of 96%. For KIBs, the HCONs also deliver a high-rate capability of 105 mA h g−1 at 10 A g−1 and an excellent long-term cycling performance of 132 mA h g−1 at 2 A g−1 after 5000 cycles. The outstanding electrochemical performance can be ascribed to the interconnected carbon onions constructing a unique 2D nanosheet structure. On the one hand, the small graphitic layer size of carbon onions together with the abundance of N atoms can greatly enhance the ion diffusion and capacitive effects as supported by kinetic analysis. On the other hand, the hollow carbon onions possess highly stable structures that can mitigate the volume strain induced by the Na- and K-ion intercalation process. The results shed light on the further design of graphitic carbon towards favourable Na- and K-ion storage.

Carbon-coated Fe7S8 composite nanosheets with a two-dimensional (2-D) core/shell structure were synthesized by a facile pyrolysis method based on an oriented attachment growth mechanism. The composite nanosheets have an ultrathin thickness of only ca. 5 nm. When used as anode materials for SIBs, the composite nanosheets exhibited excellent sodium ion storage performances in both ether- and ester-based electrolytes. At 100 mA g−1, the composites delivered a high reversible specific capacity of 540.7 mA h g−1 with an outstanding initial coulombic efficiency (ICE) of over 90% in the ether electrolyte, and a reversible capacity of 398.6 mA h g−1 with an ICE of 65.8% in the ester electrolyte. Even at 5 A g−1, the composites still delivered capacities of 346.2 mA h g−1 in the ether electrolyte and 272 mA h g−1 in the ester electrolyte. Remarkably, the composites exhibited ultralong cycling stability at high current density in the ether electrolyte. The reversible capacities of the composite nanosheets could be maintained at 310.2 mA h g−1 at 5 A g−1 after 2000 cycles, 243 mA h g−1 at 10 A g−1 after 4600 cycles and 170.0 mA h g−1 at 20 A g−1 after ultralong cycling for 8000 cycles, respectively. The excellent electrochemical performances should be attributed to the unique carbon-coating 2-D core/shell structure of the sample. The method for preparation of the unique structure is also expected to be extended to synthesize other high-performance metal sulfide electrode materials for Na-ion storage.

Na-ion storage in graphitic carbons is a kinetically unfavorable process. It is envisioned that decreasing the size of graphitic layers can efficiently enhance Na-ion storage by shortening the diffusion pathway. However, because of the lack of model graphitic carbons, investigation on the effect of decreasing the graphitic layer size on Na-ion storage has not yet been carried out. In this work, quadrangular carbon nanotubes (q-CNTs) with herringbone-like graphitic walls are employed as model materials to exhibit the idea above. The q-CNTs show reduced dependence on the electrolytes. They deliver high capacity, excellent rate performance, and ultralong cyclic stability in both ether-based and ester-based electrolytes. Typically, the q-CNTs-600 exhibits high reversible capacities of 212 and 200 mA h g–1 at 0.1 A g–1 in ether-based and ester-based electrolytes, respectively. Even at 5 A g–1, the reversible capacity of 132 mA h g–1 can still be maintained in the ether-based electrolyte. The excellent Na-ion s...

Metal-organic frameworks can provide excellent templates for the preparation of nanomaterials for green energy storage. However, it is still a formidable challenge for the synthesis of MOF-templated Sn-based nanomaterials with controllable morphologies and structures. Here, Sn-based MOFs with different morphologies are synthesized by a facial and controllable method, then SnO/C polyhedrons with shapes of cube and stellated octahedron are obtained using the Sn-based MOFs as template by further heat treatment, respectively. Benefited from the composite structures, the SnO/C polyhedrons exhibit excellent electrochemical performances in lithium storage. A high reversible capacity of 950 mA h g−1 is obtained at a current density of 50 mA g−1 after 100 cycles, together with superior cyclic stability and remarkable rate capacity. This paper provides an effective route to prepare high-performance Sn-based anode materials from the morphology-controlled MOFs for lithium-ion batteries.

Sodium-ion battery (SIB) has been a promising alternative for sustainable electrochemical energy-storage devices. However, it still needs great efforts to develop electrode materials with ultrafast gravimetric and volumetric Na-storage performance, due to difficult balance between Na-ion diffusion kinetics and pressing density of materials. In this work, Bi2Se3/graphene composites, synthesized by a selenization reaction, are investigated as anode materials for SIBs. Na-ion storage mechanism of Bi2Se3 should be attributed to a combined conversion–alloying one by a series of ex situ measurements. In the composites, Bi2Se3 particles with an average diameter of 100 nm are uniformly dispersed onto graphene with strong interfacial interaction. Despite their nanoscale size, the pressing density of Bi2Se3/graphene composite could still reach a high value of 2.07 g/cm3. Therefore, the composites can deliver a high gravimetric specific capacity of 346 mAh/g and volumetric specific capacity of 716 mAh/cm3 at a curre...

Designing transitional metal selenides (TMSes) with superior rate and cyclic performance for sodium-ion storage remains great challenges. To achieve this task, the influence of surface oxides on Na-ion storage behavior of FeSe2 is investigated by designing FeSe2 with varying oxide content. It is found that surface oxide has an inhibitory effect on the activity of FeSe2. Small-sized FeSe2 on graphene with higher surface oxide content exhibits obviously inferior performance compared to large-sized FeSe2 with lower oxide content. By controlling oxide content, the prepared FeSe2 nanorods/graphene exhibits a high capacity of 459 mAh/g at 0.1 A/g and superior rate performance. Only 10% capacity decrease occurs with the increase in current density from 0.1 to 5 A/g. Even at 25 A/g (∼50 C), it delivers a capacity of 227 mAh/g with almost no decay after 800 cycles. The influence mechanism of surface oxide is investigated. The oxide can be converted to a sodiated shell with high mechanical strength and poor conductivity, which generates phase-transition resistance to suppress the sodiation of FeSe2 core, blocks the transfer of Na-ions and electrons in subsequent sodiation processes. Understanding the effect of surface oxide on Na-ion storage will be helpful in designing TMSes and other active materials.

Self-assembled hierarchical CuO nanostructures with fractal structures were prepared by a mild method and exhibited excellent lithium storage properties, certain of which even demonstrated a high reversible capacity of 827 mAh g−1 at a rate of 0.1 C. An interesting phenomenon was observed that the electrochemical performance varies along with the structure complexity, and the products with higher surface factal dimensions exhibited larger capability and better cyclability. Structural and electrochemical analysis methods were used to explore the lithiation kinetics of the samples and the reasons for the outstanding electrochemical performances related to the complexities of hierarchical nanostructures and the irregularities of surface and mass distribution.

Although nucleation and crystallization in solution-processed materials synthesis is a natural phenomenon, the morphology design of graphene nanosheets by controlling the dual crystallization has not been established. In this work, we systematically demonstrate how the dual crystallization of ice and potassium chloride induces the morphological variation of the freeze-dried scaffold from fractal structure toward stepped sheet-like structure. A denim-like graphene nanosheet (DGNS) has been fabricated by annealing the F127-coated stepped sheet-like scaffold in nitrogen. DGNS shows parallel and straight stripes with an average stripe spacing of 10 nm. When used as a lithium-ion battery anode, DGNS possesses a superhigh reversible capacity of 1020 mAh g–1 at the current density of 1 A g–1 after 600 cycles. This work reports the control of dual crystallization of ice and salt crystals and provides an efficient way to design the morphology of two-dimensional materials by adjusting the crystallization.

The exploration of new anode materials with high gravimetric and volumetric Na-ion storage performances is urgent and advantageous, but it remains a great challenge. In this study, CoTe2/G composite nanosheets are prepared to achieve this goal by a solvothermal method due to the good properties and high density of 7.92 g cm−3 of bulk CoTe2. In the composite, CoTe2 nanoparticles with a size of 39 nm are uniformly anchored on graphene with strong interfacial interactions. The tap density of the composite can be as high as 1.82 g cm−3. As an anode material for SIBs, the CoTe2/G composite exhibits a high initial reversible gravimetric capacity of 382 mA h g−1 and a volumetric capacity of 695.2 mA h cm−3 along with excellent rate performance. Even at 1, 2 and 5 A g−1, the CoTe2/G composite still delivers gravimetric capacities as high as 295, 272 and 246 mA h g−1 and volumetric capacities of 536.9, 495.1 and 447.7 mA h cm−3. After 1000 cycles at 2 A g−1, the reversible gravimetric and volumetric capacities remain at 245 mA h g−1 and 445.9 mA h cm−3, respectively. The electrochemical dynamic analyses indicate that the Na-ion storage behaviour in the CoTe2/G composite is a surface-controlled capacitive process, which is helpful for higher rate performance. Besides, the enhanced performance of the CoTe2/G anode is ascribed to strong interfacial interaction between CoTe2 and graphene, which can effectively improve electron transfer and alleviate the volume changes of CoTe2 during the discharge/charge processes in SIBs. Accordingly, the CoTe2/G composite has great application prospect as a promising anode candidate for smart SIBs.

A free-standing Co0.85Se nanosheets/graphene (Co0.85Se NSs/G) composite film was developed as a high performance anode material for both lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) by a simple vacuum filtration and thermal reduction process. The Co0.85Se NSs/G film demonstrates good flexibility, and the Co0.85Se NSs are uniformly anchored on the graphene to form “sheet-on-sheet” nanostructures with strong interfacial interactions. The hybrid lamellar film demonstrated excellent electrochemical performances when used as a binder-free anode for LIBs and SIBs. As an anode material for LIBs, the Co0.85Se NSs/G film exhibited a high initial reversible capacity of 680 mA h g−1 at 50 mA g−1 and an excellent cycling stability and rate performance. The specific capacity of the Co0.85Se NSs/G film shows almost no fading from 0.05 to 0.5 A g−1, and the capacities were maintained at 613.3 and 522.7 mA h g −1 at 1 and 2 A g−1, respectively. When used as an anode for SIBs, the reversible capacity of the Co0.85Se NSs/G film reached up to 388 mA h g−1 with an initial coulombic efficiency as high as 82.5% at 50 mA g−1. Even at the higher current densities of 1 and 2 A g−1, the reversible capacities remained at 137.5 and 112.4 mA h g−1 after 500 cycles, respectively. The excellent electrochemical performances should be attributed to the special structure of the Co0.85Se NSs/G film. On one hand, graphene not only provides an efficient electrically conductive network, but also acts as an elastic layer to buffer the volumetric expansion of the Co0.85Se NSs during the discharge/charge process in LIBs/SIBs, which is helpful for enhancing the cycling stability of the Co0.85Se NSs/G film. On the other hand, strong interfacial interactions can further enhance electron transfer between Co0.85Se NSs and graphene, which is beneficial for improving the cycling stability and rate-performance of the Co0.85Se NSs/G film.

To improve the electrochemical performance of pliable graphene/metal oxide composite papers, a new kind of embedded-type MO@PG paper electrode including Fe2O3@PG and CuO@PG was designed and prepared by simple filtration and controllable oxidation. MO@PG paper electrodes have special structures in which hollow metal–oxide nanoparticles are embedded in the pores of porous graphene films and connected with the edges of the graphene to produce a strong interfacial interaction. Benefiting from this unique structure, the MO@PG paper electrodes for LIBs exhibit higher specific capacities and better cyclic stability and rate performance. In particular, the MO@PG electrodes show an interestingly abnormal rate performance in that the specific capacity increases with the current density from 50 to 500 mA g–1, which should be attributed to Li-ion storage in graphene films being mainly controlled by diffusion, while electrochemical reactions of Fe2O3 are mainly controlled by Faradaic capacitance. At a higher current d...

With the objective of improving the capacity and cycling stability of SnO 2 -based anode materials, a new material of SnO 2 /TiO 2 nanocomposites dispersed at porous carbon was synthesized via a route of absorption and carbonization by using Ti-MOF (MIL-125) as both titanium and carbon sources and SnCl 2 as tin source. The composite shows micro-/nano-scale combined configuration: a monodisperse sub-micro cylinder structure formed by well-dispersed SnO 2 and TiO 2 nanoparticles in amorphous carbon matrix. Benefiting from the synergistic effect of TiO 2 nanoparticle and amorphous carbon on maintaining the robust architecture as well as the uniform and stable nano-sized particles, the composite exhibits excellent performance as anode material for lithium-ion batteries: a high specific capacity of 1177 mA h g −1 and 88.7% capacity retention after 100 cycles at 100 mA h g −1 , and a stable capacity of 487 at high current density of 2 A g −1 .

Chrysanthemum-like carbon/FeS microspheres (CL-C/FeS) are prepared from a one-step solvothermal method. The morphology and structure of the CL-C/FeS are characterized by X-ray diffraction, scanning electron microscopy and high-resolution transmission electron microscope. We note that the prepared CL-C/FeS microspheres exhibit the average diameter of 15 μm and possess a sheet-on-sheet structure. FeS nanosheets are stacked on the carbon sheets to form a 2D sheet-on-sheet structure, these 2D C/FeS nanosheets are self-assembled to form a 3D chrysanthemum-like morphology. Due to the unique structure, CL-C/FeS microspheres show the excellent sodium-storage performance of 500 mAh g−1 at 0.2 A g−1 and 260 mAh g−1 at 1 A g−1, which are higher than those of most reported values. Therefore, the CL-C/FeS with appropriate structure is expected to be a competitive choice for anode materials for sodium ion batteries.

Carbon-coated Li 4 Ti 5 O 12 composites (LTO/C) with tablet-like morphology are firstly in-situ synthesized from lithium-doped Ti-based metal-organic framework precursor by thermal annealing in N 2 atmosphere at 800 and 900 °C, denoted as LTO/C-800 and LTO/C-900, respectively. The TEM and TG results demonstrate that LTO is embedded in carbon uniformly and LTO/C-800 presents more carbon content than LTO/C-900. The electrochemical results show that the LTO/C electrodes deliver a high reversible capacity, high-rate performance and excellent cycling stability as lithium-ion battery anodes. At the current density of 500 mA g −1 , the reversible capacities of LTO/C-900 and LTO/C-800 are 277 and 223 mA h g −1 after 700 cycles, respectively. At 1000 and 2000 mA g −1 , the reversible capacities of LTO/C-900 and LTO/C-800 can reach 181, 150 mA h g −1 and 174, 88 mA h g −1 after 1000 cycles with no attenuation, respectively. The facile and economical synthesis strategy will extend the scope of lithium-doped MOFs synthesis for other materials in energy storage application.

A cross-linking strategy is proposed to design electrospun carbon nanofiber films towards free-standing and binder-free electrodes with high sodium-ion storage performance. Based on this strategy, electrospun cross-linked carbon nanofiber (CL-CNF) films are fabricated with polyvinylpyrrolidone solutions containing Cu(NO3)2. The formation of the cross-linked structure should be ascribed to the strong coordination of PVP with Cu(NO3)2, which acts as a cross-linking agent. The as-prepared CL-CNF film is flexible, and the average diameter of the nanofibers can be facilely adjusted by controlling the feeding rate. Remarkably, the CL-CNF film demonstrates an excellent rate performance and long cycle stability when used as a binder-free anode for SIBs, compared with the CNF film without a cross-linked structure. At 50 mA g−1, the CL-CNF film delivered a specific capacity of as high as 449 mA h g−1, which can rival the Na ion storage capacity of most of the reported electrospun nanofibers. At 5 and 10 A g−1, the CL-CNF film delivers an initial reversible capacity of 148 and 121 mA h g−1, respectively, and still maintains 126 and 111 mA h g−1 after 500 cycles without obvious capacity fading. The excellent electrochemical properties of the CNF film are attributed to the unique cross-linked structure that endows the CL-CNF film with fast electron transfer and ion diffusion kinetics as well as robust structural stability to bear the repeated impact of Na ions during the discharge/charge process. Therefore, this work opens up a new strategy to design high performance carbon nanofibers as free-standing electrodes for flexible energy sodium-ion storage devices.

The optional molecular structures and compositions have made metal–organic frameworks (MOFs) an important precursor for preparing functional nanomaterials. In this paper, ZnO nanosheets growing on a MOF-derived porous carbon matrix (ZnO/MPC) were synthesized by in situ pyrolysis of a Zn-based mixed-ligand MOF. The formation of ZnO nanosheets relies on the intermediate structures of the pyrolytic MOFs. The inherent molecular structure and the escape of the ligand molecules will induce the formation of a stacked structure during the heat treatment process, and the layered spaces can provide a directional path for the motion of Zn atoms to the surface of carbon bulk during the pyrolysis of the precursor. This work shows that one can design and synthesize novel nanostructures by controlling the intermediate structures of MOF precursors during the pyrolysis process. The as-synthesized ZnO/MPC also displayed an excellent cyclability and a high reversible capacity of 920 mA h g−1 at a current density of 60 mA g−1, exhibiting a promising prospect in lithium storage application.

For improving the capacity and stability of Sn-based anode materials, a novel Sn–Co nanoalloy embedded in porous N-doped carbon was synthesized using the metal–organic framework ZIF-67 as both the template and carbon source, and SnCl4 as the tin source through carbonization. This composite shows the shape of a microbox with the diameter of about 2 μm in which about 10 nm of Sn–Co nanoalloy particles were uniformly embedded. When used as the anode material for lithium-ion batteries, it exhibits a high capacity of 945 mA h g−1, and 86.6% capacity retention after 100 cycles at 100 mA g−1 as well as an excellent rate capacity of 472 mA h g−1 at a high current density of 2 A g−1. The superior electrochemical performance can be ascribed to the well-dispersed, nano-sized alloy and the buffering effect of porous N-doped carbon coating. Moreover, the uniform particles remain intact upon cycling which gives the material enhanced electrochemical stability.

Heteroatom-doping is an effective way to regulate the electronic structure of carbon so as to improve their Na-ion storage capability. In all kinds of heteroatoms, selenium (Se)-doping has rarely been investigated even though Se has unique physicochemical properties. Herein, we report a facile fabrication of Se/N-codoped carbon nanosheets (N/Se-CNs) with 10 at% N and 2 at% Se by an oxidation-selenization process. The synchrotron X-ray photoelectron spectroscopy (XPS) measurements indicate that Se doping can significantly change the electronic structures of carbon nanosheets, because Se can act as stronger electron-withdrawing units for Na storage compared with C and N. Therefore, Se can better play the role of improving the conductivity and wettability of the carbon materials. And Se atoms effectively enlarge the interlayer distance of carbon nanosheets to enhance the Na-ion diffusion kinetics and also generate extra active sites for Na-ion storage. Served as anode materials for SIBs, the N/Se-CNs delivers higher specific capacity and better rate capability compared with only N-doped carbon nanosheets. Therefore, Se-doping is an effective way for improvement of Na-storage capability of carbon materials. Moreover, this work should provide a new pathway for design of carbons doped by heteroatoms with large atomic size towards excellent Na-ion storage.

Porous carbon (PC) materials with a large specific surface area and high volume of mesopores were produced using iron-doped calcium tartrate (CaTr) as a template precursor and ethanol as a carbon source. The iron doping concentration was varied from 0.2 to 1.6 at%. Template precursor was decomposed in argon at 800 °C, the formed template nanoparticles were immediately used for chemical vapor deposition (CVD) of ethanol, and after the synthesis, they were removed by HCl treatment. It was shown that the template consisted of CaO nanoparticles 5–20 nm in size with oxidized carbon layers on the surface. The incorporation of iron into CaTr promoted graphitization of the carbon layers and led to formation of iron oxide nanoparticles 3–4 nm in size. These nanoparticles served as a replica for small mesopores in the graphitic layers obtained in the ethanol CVD process. PC materials grown on the templates with an iron doping 0.2–0.4 at% were enriched with small mesopores, and due to this, they had a surface area of 907–931 m2 g−1 and pore volume of 3.3 cm3 g−1. These materials showed the highest specific capacitances of 280–230 F g−1 at 2 mV s−1 in 1 M H2SO4 electrolyte and a good rate capability.

With the increasing use of sodium-ion batteries (SIBs), developing cost-effective anode materials, such as metal oxide, for Na-ion storage is one of the most attractive topics. Due to the obviously larger ion radius of Na than that of Li, most metal oxide electrode materials fail to exhibit the same high performance for SIBs like that of Li-ion batteries. Herein, iron oxide was employed to demonstrate a concept that rationally designing an amorphous structure should be useful to enhance Na-ion storage performance of a metal oxide. Amorphous Fe2O3/graphene composite nanosheets (Fe2O3@GNS) were successfully synthesized by a facile approach as anodes for SIBs. It reveals that amorphous Fe2O3 nanoparticles with an average diameter of 5 nm were uniformly anchored on the surface of graphene nanosheets by the strong C–O–Fe oxygen-bridge bond. Compared to well-crystalline Fe2O3, amorphous Fe2O3@GNS exhibited superior sodium storage properties such as high electrochemical activity, high initial Coulombic efficienc...

To investigate the pore structural effects on the electrochemical performance of ordered mesoporous carbons (OMCs) as anode materials for sodium ion battery (SIB), we prepared OMCs with various rod lengths from 350 nm to 1300 nm, and different pore sizes from 4.7 nm to 6.5 nm by changing the hydrochloric acid concentration in the P123/silica/glycerol composite system. The reversible capacities of OMCs with the average length of 350 nm, 700 nm, 900 nm and 1300 nm were 214 mA h g −1 , 217.4 mA h g −1 , 232.6 mA h g −1 , and 228.9 mA h g −1 at 50 mA g −1 , respectively. Furthermore, the OMC with largest pore size (6.5 nm) presented the highest capacity and even remained 100 mA h g −1 after 1000 cycles at 500 mA g −1 with the coulombic efficiency of nearly 100%. We confirmed that the carbon with ordered mesostructure, bigger pore size and shorter length of pore channel exhibited higher capacity. The results propose an effective direction and strategy for designing mesoporous materials to enhance the electrochemical performance of SIB.

Novel branched carbon encapsulated MnS (MnS@C) nanochains were prepared by an in situ co-pyrolysis method. The morphology and structure of the MnS@C nanochains were mainly characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). It was found that the prepared MnS@C nanochains possess interesting branched structures, which are constructed by interconnected MnS@C nanoparticles with a diameter of ca. 200–400 nm. More interestingly, the MnS@C nanoparticles have novel “pomegranate-like” structures, in which inner cores are not made of whole nanoparticles but composed of many MnS nanoparticles. The formation mechanism of MnS@C should be attributed to an Oriented Attachment (OA) mechanism by investigating various intermediate products obtained by controlling the reaction conditions. The branched MnS@C nanochains after annealing (MnS@C-800) demonstrated perfect cycling stability and long cycle life when used as anode materials for lithium-ion batteries (LIBs). At a current density of 50 mA g−1, the stable specific capacity is around 545 mA h g−1 while the pure MnS anode experiences a drastic drop quickly to 300 mA h g−1 at the initial few cycles. At 500 mA g−1, the reversible specific capacity is ca. 318 mA h g−1 at the initial cycle and is maintained at ca. 200 mA h g−1 after 800 cycles.

We develop a general strategy for the design of a series of 2D N-rich carbon-based nanomaterials through the thermal exfoliation of layered metal-hexamine framework microcrystals. By a facile pyrolysis, the sharply generated gases escape from the layered precursors, leading to the exfoliation of the layers and successful preparation of 2D carbon-based nanomaterials. This strategy can readily skip the complicated morphology engineering process of 2D metal-organic frameworks to produce 2D N-rich carbon-based nanomaterials on a large scale.

This article has been retracted: please see Elsevier Policy on Article Withdrawal (http://www.elsevier.com/locate/withdrawalpolicy).This article has been retracted at the request of the Authors. The authors deeply apologize that due to their misunderstanding of the nature of the conference proceedings of WCPT7, this article became a duplicate of a paper that has already been published in RSC Adv., (2014), 4, 29435-29438. DOI: 10.1039/C4RA02236J.One of the conditions of submission of a paper for publication is that authors declare explicitly that the paper is not under consideration for publication elsewhere. As such this article represents a severe abuse of the scientific publishing system.The scientific community takes a very strong view on this matter and apologies are offered to readers of the journal that this was not detected during the processing of this submission.

NH2-terminated Si nanoparticles with an ultrathin silica shell have been efficiently obtained by a one-step reaction in ammonia–water–ethanol solution. Graphene nanosheet (GNS) encapsulated Si@ultrathin SiOx has been fabricated by self-assembly and thermal treatment. Because of the uniform ultrathin SiOx shell and superior GNS encapsulation structure, this material shows a reversible capacity of 2391.3 mA h g−1, maintaining 1844.9 mA h g−1 after 50 cycles at a current density of 200 mA g−1, and good rate and long cycle performance (∼700 mA h g−1 at 2000 mA g−1 after 350 cycles) as well.

Novel hierarchical porous carbon microspheres (HPCM) with quantities of micropores and mesopores have been prepared by an alcohol-in-oil emulsion technique using thermoplastic phenolic formaldehyde resin (PF) as the carbon source and copper nitrate (CN) as the template precursor. The effects of CN loading content on the morphology and structure of HPCM were investigated. The results showed that, when the mass ratio of PF and CN is 1:4, the HPCM not only can maintain hierarchical porous microsphere structure, but also display high electrochemical performance with a reversible capacity of 585 mA h g−1 at a current density of 50 mA g−1 and favorable high-rate performance when used as the anode materials for lithium-ion batteries.

Porous carbon is one of the most promising alternatives to traditional graphite materials in lithium-ion batteries. This is not only attributed to its advantages of good safety, stability and electrical conductivity, which are held by all the carbon-based electrodes, but also especially ascribed to its relatively high capacity and excellent cycle stability. Here we report the design and synthesis of a highly porous pure carbon material with multifractal structures. This material is prepared by the vacuum carbonization of a zinc-based metal-organic framework, which demonstrates an ultrahigh lithium storage capacity of 2458 mAh g−1 and a favorable high-rate performance. The associations between the structural features and the lithium storage mechanism are also revealed by small-angle X-ray scattering (SAXS), especially the closed pore effects on lithium-ion storage.

Application of iron fluoride, a promising candidate of cathode materials for lithium ion batteries, is being hindered by its poor electrochemical performance caused by low electronic conductivity and large volume change. Design of carbon-encapsulated transitional metal compounds (including fluoride, oxide, sulfide, etc.) structure is one of the most effective strategies in improving their lithium-ion storage performance. In this work, we successfully synthesize for the first time carbon-nanotube-encapsulated FeF2 nanorods via a facile in situ co-pyrolysis of ferrocene and NH4F. This kind of core/shell carbon nanotube/FeF2 nanorod exhibits better cyclic stability and rate-performance used as cathode materials. Better electrochemical performance of the nanorods should be attributed to the protection of the carbon shell because, experimentally, it is observed that outer carbon shells suffer from high internal stress during Li-ion insertion but efficiently keep the nanorods in the one-dimensional morphology a...

Generally speaking, excellent electrochemical performance of metal oxide/graphene nanosheets (GNSs) composite is attributed to the interfacial interaction (or "synergistic effect") between constituents. However, there are no any direct observations on how the electronic structure is changed and how the properties of Li-ion storage are affected by adjusting the interfacial interaction, despite of limited investigations on the possible nature of binding between GNSs and metal oxide. In this paper, CuO nanosheets/GNSs composites with a little Cu2O (ca. 4 wt %) were utilized as an interesting model to illustrate directly the changes of interfacial nature as well as its deep influence on the electronic structure and Li-ion storage performance of composite. The interfacial adjustment was successfully fulfilled by removal of Cu2O in the composite by NH3·H2O. Formation of Cu-O-C bonds on interfaces both between CuO and GNSs, and Cu2O and GNSs in the original CuO/GNSs composites was detected. The small interfacial alteration by removal of the little Cu2O results in the obvious changes in electronic structure, such as weakening of covalent Cu-O-C interfacial interaction and recovery of π bonds in graphene, and simultaneously leads to variations in electrochemical performance of composites, including a 21% increase of reversible capacity, degradation of cyclic stability and rate-performance, and obvious increase of charge-transfer resistance, which can be called a "butterfly effect" in graphene-based metal oxide composites. These interesting phenomena could be helpful to design not only the high-performance graphene/metal oxide anode materials but also various advanced graphene-based composites used in the other fields such as sensors, catalysis, fuel cells, solar cells, etc.

Cobalt hydroxide arrays/graphene nanosheets (Co(OH)2 array/GNSs) composites are formed by the growth of preferential-orientation versus basal planes of GNSs. Notably, the Co(OH)2 nanoplates, possessing special hexagonal morphology with the length of about 130 nm and average thickness of about 20 nm, stand vertically rather than lie flat on the surface of graphene sheets. More interestingly, the (001) crystal plane of Co(OH)2 is vertical to the graphene basal plane and forms strong covalent bond interaction (Co–C bond) with the graphene layer, which could be a driving force for the preferential growth of nanoarrays on graphene. Co(OH)2 array/GNSs composites exhibit not only high specific capacity, but also outstanding high-rate performance when used as anode materials for lithium-ion batteries. At the current density of 50 mA g−1, the reversible capacity is 976 mA h g−1 and remains at 1103 mA h g−1 after 50 cycles without any fading. At 500 and 1000 mA g−1, the reversible capacities can reach 696 and 496 mA h g−1, which are 71% and 51% of that at 50 mA g−1, respectively. Excellent electrochemical performance could be attributed to the array structure of Co(OH)2 as well as a synergistic effect between Co(OH)2 and graphene. Therefore, Co(OH)2 array/GNSs composites are expected to have potential applications in LIBs.

Si@SiO2 core–shell, yolk–shell, and SiO2 hollow structures can be obtained when Si nanoparticles are simply treated with ammonia–water–ethanol solution at room temperature. Their formation mechanism is attributed to the self-templated etching–deposition process.

The confinement of Sn nanoparticles inside few-walled carbon nanotubes and its effect on lithium ion storage property have been investigated in detail. It was found that the charge transfer and electronic interaction facilitated Sn to remain in a more reduced state and to link strongly with interior surface of carbon nanotubes by Sn–C bonds, leading to a high reversible capacity of 732 mAh g–1 and capacity retention of 639.7 mAh g–1 after 170 cycles at 50 mA g–1. We also found that the volume restriction inside CNTs protected Sn–C bonds against breakage during lithium insertion/extraction, contributing to the excellent cycling performance with the fade rate of only 0.074% per cycle.

A novel porous nanoarchitecture composed of 2D graphene–SnS2 (G–SnS2) units is developed via a two-step approach in this work. The special structure endows the high-rate transportation of electrolyte ions and electrons throughout the electrode matrix, resulting in remarkable electrochemical performance when it was used as anode in lithium ion batteries.

Copper oxide hollow nanoparticles/graphene-nanosheet composites are prepared using the Kirkendall-effect approach. The composites exhibit a durable lifetime cycle at high rates. The reversible capacity of the material attains 640 mAhg−1 at 50 mAg−1 and the capacity retention is ca. 96% when the current density is increased 10 times. At 1 Ag−1 (ca. 1.7 C), the reversible capacity reaches 485 mAhg−1 and remains at 281 mAhg−1 after 500 cycles, indicating that the capacity fading is less than 0.4 mAhg−1 per cycle. This excellent electrochemical performance can be attributed to the hollow interior of CuO nanoparticles as well as synergistic effect between CuO and graphene. Keywords: Copper oxide, Hollow nanoparticles, Graphene, Anode, Lithium-ion batteries

Graphite oxide was prepared by the Hummers method. Then after further oxidation, a new kind of carbon nanoparticle, with diameter 10–30 nm, was formed in the aqueous solution. On the basis of structural characterization by X-ray diffraction, Fourier transform infrared spectroscopy, and transmission electron microscopy it is deduced that the nanoparticles are generated by the self-assembly of few-layer graphene oxides. A possible formation mechanism is proposed.

To prepare highly N-doped carbon materials (HNCs) as well as to determine the influence of N dopants on Na-ion storage performance, hexamine-based metal-organic frameworks are employed as new and efficient precursors in the preparation of HNCs. The HNCs possess reversible capacities as high as 160 and 142 mA h g-1 at 2 A g-1 (≈8 C) and 5 A g-1 (≈20 C), respectively, and maintain values of 145 and 123 mA h g-1 after 500 cycles, thus exhibiting excellent rate and long-term cyclic performance. Based on systematic analysis, a new insight into the roles of the different N configurations in Na-ion storage is proposed. The adsorption of Na ions on pyridinic-N (N-6) and pyrrolic-N (N-5) is fully irreversible, whereas the adsorption on graphitic-N (N-Q) is partially reversible and the adsorption on N-oxide (N-O) is fully reversible. More importantly, the N-6/N-Q ratio is an intrinsic parameter that reflects the relationship between the N configurations and carbon textures for N-doped carbons prepared from in situ pyrolysis of organic precursors. The cyclic stability and rate-performance improve with decreasing N-6/N-Q ratio. Therefore, this work is of great significance for the design of N-doped carbon electrodes with high performance for sodium ion batteries.

We synthesized α-Fe2O3 hollow nanoparticles by directly oxidizing the carbon-encapsulated iron carbide (Fe3C@C) nanoparticles in air. In this paper, the conversion mechanism of Fe3C@C to hollow nanoparticles was deduced in detail by comparatively investigating the morphologies and compositions of the oxidized products at different oxidation stages using transmission electron microscope (TEM), high resolution TEM (HRTEM), energy-dispersive X-ray (EDX), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). It was found that both oxygen and carbon play important roles in the formation of hollow nanostructures, wherein oxygen is the driving force for the outward diffusion of core species and the carbon shell not only provides the diffusion vacancies but also effectively moderates the interdiffusion rates of metal core materials and oxygen. A growth model was proposed: during the oxidation process, three diffusion processes occur including the inward diffusion of oxygen along the carbon shell, o...

A porous magnetic hollow silica nanosphere (MHSN) is a new nanostructured drug carrier for increasing drug loading capability. Keeping the magnetic nanoparticles in the hollow core will limit the toxicity and degradation in a biosystem. In this paper, we report a synthesis of porous MHSNs by sol−gel method. CaCO3/Fe3O4 composite particles were first fabricated by embedding Fe3O4 nanoparticles into CaCO3 using the rotating packed bed (RPB) method. Tetraethoxysilane (TEOS) was then added as precursor to form a silica (SiO2) layer on the surface of CaCO3/Fe3O4 composite particles. Hexadecyltrimethylammonium bromide (CTAB) and octane act as second templates for the formation of porous silica shells. After removing the surfactants by calcination and etching away the CaCO3 particles, porous MHSNs with magnetite (Fe3O4) nanoparticles inside the cores were formed. The pore size can be tuned by adjusting the amount of the cationic surfactant absorbed on the surface of the composite particles to form self-assembled...

We have fabricated CuO@TiO2 nanocable arrays by a facile method involving in situ thermal oxidation of Cu foil and coating of tetrabutyl titanate solution. The structure of the nanocables has been investigated by various techniques to comfirm that the cores are mainly crystalline monoclinic CuO, and the shells are crystalline tetragonal anatase TiO2. When used as an anode material for lithium-ion batteries, the nanoconfinement effect plays an important role in improving the lithium-ion storage preformance: the lithiation will be confined in one-dimensional space of TiO2 nanotubes to limit the pulverization of CuO, and the phase interface will cause an interfacial adsorption to enrich more lithium ions at some level. Benefiting from the nanoconfinement effect and interfacial adsorption, the reversible capacity does not fade, but rather increases gradually to 725 mAh g(-1) after 400 cycles at a current density of 60 mA g(-1), superior to the theoretical capacity of CuO.