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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.

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 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.

In this work, the triblock copolymer F127 has been used as the carbon precursor to fabricate carbon nanosheets (CNSs) by a sodium chloride surface-assisted bottom-up strategy. The CNSs possess a thickness of about 6 nm and an oxygen content of 15.6 at%. As an anode for lithium-ion batteries, CNSs exhibit reversible capacities of 830 mA h g−1 after 500 cycles at 1 A g−1 and 240 mA h g−1 at 20 A g−1, which is among the best of the reported lithium storage performances of carbon materials. When used as an anode material in sodium-ion batteries, the CNS electrode exhibits a reversible capacity of 367 mA h g−1 at 50 mA g−1 after 60 cycles and still delivers 112 mA h g−1 at 20 A g−1. The high reversible capacity and excellent rate performance would be ascribed to the synergistic effect of the 2D structure, defects, and expanded crystallites. This work extends the sodium chloride template method to prepare thin CNSs by using a suitable carbon precursor and provides some insights into the relationship between the structure of CNSs and the electrochemical performance.

Transition metal fluorides (TMFs) have received increasing attention as promising electrode materials for lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) due to their resource abundance, low-cost and high specific capacities. However, TMFs usually suffer from low electron conductivity and high Li/Na ion diffusion resistance, which lead to rapid capacity fading. In order to further improve their electrochemical performance, designs of carbon-based TMFs with crystal topologies favorable for Li/Na ion diffusion are greatly needed. Here, NH4FeF3/carbon nanosheet (NH4FeF3/CNS) composites were prepared via a facile co-pyrolysis of ferric acetylacetonate and NH4F. NH4FeF3 has an open framework structure with a perovskite topology, in which FeF6 octahedral monomers are connected with each other via F− anions to form cavities, and NH4+ cations reside inside the cavities. The interesting perovskite structure is favorable for Li/Na ion storage. When used as an anode for LIBs, the NH4FeF3/CNS exhibits a specific capacity of 1000 mA h g−1 at 200 mA g−1 after 300 cycles, which is much higher than those of other reported TMFs. When used as an anode for SIBs, the NH4FeF3/CNS also exhibits a high specific capacity of 504 mA h g−1 as well as better rate-performance and cycling stability. The better electrochemical performance of NH4FeF3/CNS composites for both LIBs and SIBs should be ascribed to, on one hand, the fact that the perovskite framework structure of NH4FeF3 with NH4+ fillers has kinetically favorable Li/Na ion channels so as to be helpful to alleviate volume expansion during the cycling process and, on the other hand, the fact that carbon nanosheets can act as a conductive network to improve the conductivity of NH4FeF3 nanoparticles.

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.

Porous graphene films (PGFs) were developed by introducing defects and extra edges into graphene using GO and a metal salt (ferric nitrate) as sources via a facile filtration method together with a thermal reduction and subsequent removal of the metal. The pore size and density could be controlled by simply adjusting the amount of ferric nitrate. When used as an anode for lithium ion batteries, PGF-1 showed a high reversible capacity, improved cycling stability, and ultra-high rate performance (971, 298, and 163 mA h g−1 at the rates of 10, 30, and 50 A g−1 after 10 000 cycles). When used as an anode for sodium ion batteries, PGF-1 showed a reversible capacity of 195 mA h g−1 at 50 mA g−1 after 50 cycles. Even at a high rate of 1000 mA g−1, the reversible capacity can still remain at 111 mA h g−1 after 1000 cycles. The excellent performance should be attributed to the special porous structure of the PGF. On one hand, plenty of defects within the PGF provided extra reaction sites for lithium and sodium ion storage. On the other hand, the porous structure of the PGF resulted in fast diffusion and transfer of lithium/sodium ions and electrons throughout the electrodes.

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.

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.

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.

In most of the reports, ball-milling of graphitic materials would lead to the decrease of their crystal degree and further amorphization. However, large crystal enhancement of carbon black and graphitized carbon black induced by ball milling were herein observed. The hollow structure of the graphitized carbon black remained without collapse. But from high resolution transmission electron microscopy, the morphology of each nanoparticle is changed from polyhedron to sphere after a certain time of ball-milling. Accordingly, a transformation model was proposed.

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.

We reported a universal approach to prepare carbon nanotubes from solid-state carbons. Under the assistance of a trace amount of water vapor, efficient growth of types of carbon nanotubes were achieved from commercially available artificial graphite powder, carbon black, and amorphous carbon powder under a relatively low temperature of 850 °C. This present method provided a new horizon for the effective utilization of low value added carbon resources to fabricate advanced carbon materials and could help to deeply understand the structure transformation of solid carbon materials.

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.

The coalescence and structural transformation of closed-shell graphitic carbon (CGC) induced by KOH activation were investigated by scanning electron microscopy, high resolution transmission electron microscopy, X-ray diffraction and Raman spectroscopy. It was found that with graphitization at 2800 °C, CGC retains a nanosized polyhedral structure with a diameter of ca. 25 nm. These thermally stable CGC nanoparticles could be coalesced and reconstructed to form a micron-sized dish stacked structure when treated by low temperature KOH activation at 400 °C. Each of the obtained carbon dishes retained a high crystallinity and possessed few layer graphene-like structure with closed loop edges. During activation, defects, disordered carbon atoms and dangling bonds were introduced into the CGC, which provide active positions for the coalescence and reconstruction of these nanoparticles. With the destruction of the closed-shell structure and the structural transformation, the inner-stress of these polyhedral nanoparticles was released. Accordingly, a possible formation mechanism was suggested.

Due to its inert closed-shell structure, the graphitic onion-like carbon (GOC) could not be exfoliated into a graphene-like structure by a simple Hummers' method even under rigorous conditions. Herein, we successfully exfoliated GOC into twisted graphene oxide nanosheets by the cooperation of chemical activation and Hummers' oxidation. Porphyra-like graphene nanosheets were further prepared from the twisted graphene oxide nanosheets by rapid thermal expansion. The defects introduced by activation were considered to play significant roles in the unfolding of GOC because they provide reactive sites for the oxidation. As an anode material for lithium ion batteries, the obtained porphyra-like graphene exhibited much better electrochemical properties than those of GOC. Our work can contribute to (1) deeply understand the exfoliation mechanism of graphitic materials by chemical oxidation especially the influence of defects during these steps; and (2) provide new strategies for designing functional graphene based materials from inert graphitic structures.

CuO nanowires were synthesized by in situ thermal oxidation of Cu2O for the first time in the temperature range of 300–750 °C in air. In this process, the effects of annealing temperature and growth time on the growth of CuO nanowires were investigated by SEM, XRD, EDS mapping and line-scan profile measurements. We found that the nucleation of CuO nanowires is a solid-state transformation process, and the nanowire length obeys a parabolic law with annealing time. The nanowire growth shows a diffusion-controlled behavior. Based on the above-mentioned study, the mechanism of the nucleation and growth of CuO nanowires from Cu2O is proposed.

Solution-based oxidation has been widely utilized to prepare graphene oxide or graphene nanoribbons from different carbon precursors, but some details of the exfoliation or unzipping processes still remain elusive. Here, we put forward three graphitic systems for deep understanding of the top-down route. Based on the oxidation-intercalation synergistic effect, the formation mechanisms were proposed.

Novel hierarchical porous carbon nanosheets (HPCS) with quantities of micropores and mesopores were prepared on a large-scale by using thermoplastic phenolic formaldehyde resin as the carbon source and copper nitrate as the template precursor. The HPCS, possessing a thickness of about 40 nm and the width of several microns, exhibited a high specific capacity and favorable high-rate performance when used as an anode material for lithium ion batteries (LIBs). The reversible capacities were 748 mA h g−1 at a current density of 20 mA g−1 and 460 mA h g−1 even at 1 A g−1, which were much higher than those of traditional porous carbon materials. It also showed superior cyclical stability for only 0.3% capacity loss per cycle under high rate charge-discharge process, suggesting that HPCS should be a promising candidate for anode materials in high-rate LIBs. The roles of various-sized pores in HPCS in Li storage were discussed briefly.

We explore in-depth the interfacial interaction between Fe3O4 nanoparticles and graphene nanosheets as well as its impact on the electrochemical performance of Fe3O4/graphene anode materials for lithium-ion batteries. Fe3O4/graphene hybrid materials are prepared by direct pyrolysis of Fe(NO3)3·9H2O on graphene sheets. The interfacial interaction between Fe3O4 and graphene nanosheets is investigated in detail by thermogravimetric and differential scanning calorimetry analysis, Raman spectrum, X-ray photoelectron energy spectrum and Fourier transform infrared spectroscopy. It was found that Fe3O4 nanoparticles disperse homogeneously on graphene sheets, and form strong covalent bond interactions (Fe–O–C bond) with graphene basal plane. The strong covalent links ensure the high specific capacity and long-period cyclic stability of Fe3O4/graphene hybrid electrodes for lithium-ion batteries at high current density. The capacity keeps as high as 796 mAhg−1 after 200 cycles without any fading in comparison with the first reversible capacity at the current density of 500 mAg−1 (ca. 0.6 C). At 1 Ag−1 (ca. 1.3 C), the reversible capacity attains ca. 550 mAhg−1 and 97% of initial capacity is maintained after 300 cycles. This work reveals an important factor affecting the high-rate and cyclic stability of metal oxide anode, and provides an effective way to the design of new anode materials for lithium-ion batteries.

Iron sulfide-embedded carbon microspheres were prepared via a solvothermal process and show high specific capacity and excellent high-rate performance as anode material for lithium-ion batteries.

Novel carbon nanotubes (CNTs) were prepared on a large-scale. Their morphology and structure were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and Raman measurements. It was found that the prepared CNTs possess a quadrangular cross section, as well as one open end and “herringbone”-like walls, so these novel CNTs were named q-CNTs. The unique morphology of q-CNTs implies broad potential applications in many fields, including drug delivery, conductive and high-strength composites, field emission displays and radiation sources, hydrogen storage media, and supercapacitors. When used as the anode materials for lithium-ion batteries, q-CNTs exhibit excellent high-rate performance (a high-reversible capacity of 181 mAh g−1 at the current density of 1000 mA g−1 (ca. 3 C)), which is much higher than that of the common multi-wall carbon nanotubes. This high-rate performance should be attributed to the unique nanostructure of q-CNTs, which results in a high diffusion coefficient for lithium ions in the q-CNTs.