Highly crystalline nickel hexacyanoferrate as a long-life cathode material for sodium-ion batteries

Prussian blue analogs (PBAs) are attractive cathode candidates for high energy density, including long life-cycle rechargeable batteries, due to their non-toxicity, facile synthesis techniques and low cost. Nevertheless, traditionally synthesized PBAs tend to have a flawed crystal structure with a large amount of [Fe(CN)₆]⁴⁻ openings and the presence of crystal water in the framework; therefore the specific capacity achieved has continuously been low with poor cycling stability. Herein, we demonstrate low-defect and sodium-enriched nickel hexacyanoferrate nanocrystals synthesized by a facile low-speed co-precipitation technique assisted by a chelating agent to overcome these problems. As a consequence, the prepared high-quality nickel hexacyanoferrate (HQ-NiHCF) exhibited a high specific capacity of 80 mA h g⁻¹ at 15 mA g⁻¹ (with a theoretical capacity of ∼85 mA h g⁻¹), maintaining a notable cycling stability (78 mA h g⁻¹ at 170 mA g⁻¹ current density) without noticeable fading in capacity retention after 1200 cycles. This low-speed synthesis strategy for PBA-based electrode materials could be also extended to other energy storage materials to fabricate high-performance rechargeable batteries.

A low-speed synthesis strategy was designed to fabricate Prussian blue analog based electrode materials for high-performance rechargeable batteries.     

Self-induced cobalt-derived hollow structure Prussian blue as a cathode for sodium-ion batteries

As advanced electrode materials for sodium ion batteries, Prussian blue and its derivatives have attracted considerable attention due to their low cost, structural stability and facile synthesis process. However, the application of commercially available Prussian blue is limited by its poor electronic conductivity as well as the structural defect induced by crystalline/interstitial water molecules. Herein, to address these drawbacks, an etching-agent free method is developed to synthesize Prussian blue with a hollow structure, and the synthesis mechanism is revealed. Owing to the stability of divalent iron ions, the shorter electron/ion diffusion pathway and fewer defect sites of the hollow structure, the obtained Prussian blue exhibits excellent electrochemical performance (specific capacity of 133.6 mA h g⁻¹ at 1C, 1C = 170 mA g⁻¹), which can put forward a new avenue to engineer advanced electrode materials for sodium ion batteries.

By using Fe₃[Co(CN)₆]₂ as a precursor, hollow structured Prussian blue can be synthesized via anion exchange methods without any other additive.     

3D plum candy-like NiCoMnO4@graphene as anodes for high-performance lithium-ion batteries

3D plum candy-like NiCoMnO₄ microspheres have been prepared via ultrasonic spraying and subsequently wrapped by graphene through electrostatic self-assembly. The as-prepared NiCoMnO₄ powders show hollow structures and NiCoMnO₄@graphene exhibits excellent electrochemical performances in terms of rate performance and cycling stability, achieving a high reversible capacity of 844.6 mA h g⁻¹ at a current density of 2000 mA g⁻¹. After 50 cycles at 1000 mA g⁻¹, NiCoMnO₄@graphene delivers a reversible capacity of 1045.1 mA h g⁻¹ while the pristine NiCoMnO₄ only has a capacity of 143.4 mA h g⁻¹. The hierarchical porous structure helps to facilitate electron transfer and Li-ion kinetic diffusion by shortening the Li-ion diffusion length, accommodating the mechanical stress and volume change during the Li-ion insertion/extraction processes. Analysis from the electrochemical performances reveals that the enhanced performances could be also attributed to the reduced charge-transfer resistance and enhanced Li-ion diffusion kinetics because of the graphene-coating. Moreover, Schottky electric field, due to the difference in work function between graphene and NiCoMnO_4, might be favorable for the redox activity of the NiCoMnO₄. In light of the excellent electrochemical performance and simple preparation, we believe that 3D plum candy-like NiCoMnO₄@graphene composites are expected to be applied as a promising anode materials for high-performance lithium ion batteries.

3D plum candy-like NiCoMnO₄ microspheres have been prepared via ultrasonic spraying and subsequently wrapped by graphene through electrostatic self-assembly.     

Improved pseudocapacitive charge storage in highly ordered mesoporous TiO2/carbon nanocomposites as high-performance Li-ion hybrid supercapacitor anodes

A Li-ion hybrid supercapacitor (Li-HSCs), an integrated system of a Li-ion battery and a supercapacitor, is an important energy-storage device because of its outstanding energy and power as well as long-term cycle life. In this work, we propose an attractive material (a mesoporous anatase titanium dioxide/carbon hybrid material, m-TiO₂-C) as a rapid and stable Li⁺ storage anode material for Li-HSCs. m-TiO₂-C exhibits high specific capacity (∼198 mA h g⁻¹ at 0.05 A g⁻¹) and promising rate performance (∼90 mA h g⁻¹ at 5 A g⁻¹) with stable cyclability, resulting from the well-designed porous structure with nanocrystalline anatase TiO₂ and conductive carbon. Thereby, it is demonstrated that a Li-HSC system using a m-TiO₂-C anode provides high energy and power (∼63 W h kg⁻¹, and ∼4044 W kg⁻¹).

A mesoporous TiO₂/carbon nanocomposite prepared by block copolymer self-assembly improves pseudocapacitive behavior and achieves high energy/power density Li-ion hybrid supercapacitors.     

Large-scale synthesis of ultrafine Fe3C nanoparticles embedded in mesoporous carbon nanosheets for high-rate lithium storage

Fe₃C modified by the incorporation of carbon materials offers excellent electrical conductivity and interfacial lithium storage, making it attractive as an anode material in lithium-ion batteries. In this work, we describe a time- and energy-saving approach for the large-scale preparation of Fe₃C nanoparticles embedded in mesoporous carbon nanosheets (Fe₃C-NPs@MCNSs) by solution combustion synthesis and subsequent carbothermal reduction. Fe₃C nanoparticles with a diameter of ∼5 nm were highly crystallized and compactly dispersed in mesoporous carbon nanosheets with a pore-size distribution of 3–5 nm. Fe₃C-NPs@MCNSs exhibited remarkable high-rate lithium storage performance with discharge specific capacities of 731, 647, 481, 402 and 363 mA h g⁻¹ at current densities of 0.1, 1, 2, 5 and 10 A g⁻¹, respectively, and when the current density reduced back to 0.1 A g⁻¹ after 45 cycles, the discharge specific capacity could perfectly recover to 737 mA h g⁻¹ without any loss. The unique structure could promote electron and Li-ion transfer, create highly accessible multi-channel reaction sites and buffer volume variation for enhanced cycling and good high-rate lithium storage performance.

Fe₃C modified by the incorporation of carbon materials offers excellent electrical conductivity and interfacial lithium storage, making it attractive as an anode material in lithium-ion batteries.     

Preparation of Sn-aminoclay (SnAC)-templated Fe3O4 nanoparticles as an anode material for lithium-ion batteries

Sn-aminoclay (SnAC)-templated Fe₃O₄ nanocomposites (SnAC–Fe₃O₄) were prepared through a facile approach. The morphology and macro-architecture of the fabricated SnAC–Fe₃O₄ nanocomposites were characterized by different techniques. A constructed meso/macro-porous structure arising from the homogeneous dispersion of Fe₃O₄ NPs on the SnAC surface owing to inherent NH₃⁺ functional groups provides new conductive channels for high-efficiency electron transport and ion diffusion. After annealing under argon (Ar) gas, most of SnAC layered structure can be converted to SnO₂; this carbonization allows for formation of a protective shell preventing direct interaction of the inner SnO₂ and Fe₃O₄ NPs with the electrolyte. Additionally, the post-annealing formation of Fe–O–C and Sn–O–C bonds enhances the connection of Fe₃O₄ NPs and SnAC, resulting in improved electrical conductivity, specific capacities, capacity retention, and long-term stability of the nanocomposites. Resultantly, electrochemical measurement exhibits high initial discharge/charge capacities of 980 mA h g⁻¹ and 830 mA h g⁻¹ at 100 mA g⁻¹ in the first cycle and maintains 710 mA h g⁻¹ after 100 cycles, which corresponds to a capacity retention of ∼89%. The cycling performance at 100 mA g⁻¹ is remarkably improved when compared with control SnAC. These outstanding results represent a new direction for development of anode materials without any binder or additive.

Sn-aminoclay (SnAC)/Fe₃O₄ NPs – a promising hybrid electrode to offer great electrochemical performance with a high initial discharge of 980 mA h g⁻¹ and good capacity retention of 89% after 100 cycles.     

Performance optimization of freestanding MWCNT-LiFePO4 sheets as cathodes for improved specific capacity of lithium-ion batteries

The typical lithium-ion-battery positive electrode of “lithium-iron phosphate (LiFePO₄) on aluminum foil” contains a relatively large amount of inactive materials of 29 wt% (22 wt% aluminum foil + 7 wt% polymeric binder and graphitic conductor) which limits its maximum specific capacity to 120.7 mA h g⁻¹ (71 wt% LiFePO₄) instead of 170 mA h g⁻¹ (100 wt% LiFePO₄). We replaced the aluminum current-collector with a multi-walled carbon nanotube (MWCNT) network. We optimized the specific capacity of the “freestanding MWCNT-LiFePO₄” positive electrode. Through the optimization of our unique surface-engineered tape-cast fabrication method, we demonstrated the amount of LiFePO₄ active materials can be as high as 90 wt% with a small amount of inactive material of 10 wt% MWCNTs. This translated to a maximum specific capacity of 153 mA h g⁻¹ instead of 120.7 mA h g⁻¹, which is a significant 26.7% gain in specific capacity compared to conventional cathode design. Experimental data of the freestanding MWCNT-LiFePO₄ at a low discharge rate of 17 mA g⁻¹ show an excellent specific capacity of 144.9 mA h g⁻¹ which is close to its maximum specific capacity of 153 mA h g⁻¹. Furthermore, the freestanding MWCNT-LiFePO₄ has an excellent specific capacity of 126.7 mA h g⁻¹ after 100 cycles at a relatively high discharge rate of 170 mA g⁻¹ rate.

We optimized the specific capacity of freestanding MWCNT-LiFePO₄ positive electrode. We demonstrated as high (low) as 90 wt% LiFePO₄ active material (10 wt% MWCNTs inactive material). This corresponded to a maximum specific capacity of 153 mA h g⁻¹.     

Design and synthesis of hierarchical NiO/Ni3V2O8 nanoplatelet arrays with enhanced lithium storage properties

Hierarchical NiO/Ni₃V₂O₈ nanoplatelet arrays (NPAs) grown on Ti foil were prepared as free-standing anodes for Li-ion batteries (LIBs) via a simple one-step hydrothermal approach followed by thermal treatment to enhance Li storage performance. Compared to the bare NiO, the fabricated NiO/Ni₃V₂O₈ NPAs exhibited significantly enhanced electrochemical performances with superior discharge capacity (1169.3 mA h g⁻¹ at 200 mA g⁻¹), excellent cycling stability (570.1 mA h g⁻¹ after 600 cycles at current density of 1000 mA g⁻¹) and remarkable rate capability (427.5 mA h g⁻¹ even at rate of 8000 mA g⁻¹). The excellent electrochemical performances of the NiO/Ni₃V₂O₈ NPAs were mainly attributed to their unique composition and hierarchical structural features, which not only could offer fast Li⁺ diffusion, high surface area and good electrolyte penetration, but also could withstand the volume change. The ex situ XRD analysis revealed that the charge/discharge mechanism of the NiO/Ni₃V₂O₈ NPAs included conversion and intercalation reaction. Such NiO/Ni₃V₂O₈ NPAs manifest great potential as anode materials for LIBs with the advantages of a facile, low-cost approach and outstanding electrochemical performances.

Hierarchical NiO/Ni₃V₂O₈ nanoplatelet arrays (NPAs) grown on Ti foil were prepared as free-standing anodes for Li-ion batteries (LIBs) via a simple one-step hydrothermal approach followed by thermal treatment to enhance Li storage performance.     

Magnetite ultrafine particles/porous reduced graphene oxide in situ grown onto Ni foam as a binder-free electrode for supercapacitors

Here, we report a simple and green electrochemical route to fabricate a porous network of a Fe₃O₄ nanoparticle-porous reduced graphene oxide (p-rGO) nanocomposite supported on a nickel-foam substrate, which is directly used as a binder-free charge storage electrode. Through this method, pristine Fe₃O₄ NPs/Ni, p-rGO/Ni and Fe₃O₄ NPs@p-rGO/Ni electrodes are fabricated and compared. In the fabricated Fe₃O₄ NPs@p-rGO/Ni electrode, the porous rGO sheets served as a conductive network to facilitate the collection and transportation of electrons during the charge/discharge cycles, improving the conductivity of magnetite NPs and providing a larger specific surface area. As a result, the Fe₃O₄ NPs@p-rGO/Ni exhibited a specific capacitance of 1323 F g⁻¹ at 0.5 A g⁻¹ and 79% capacitance retention when the current density is increased 20 times, where the Fe₃O₄ NPs/Ni electrode showed low specific capacitance of 357 F g⁻¹ and 43% capacity retention. Furthermore, the composite electrode kept 95.1% and 86.7% of its initial capacitances at the current densities of 1 and 4 A g⁻¹, respectively, which were higher than those of a Fe₃O₄/NF electrode at similar loads (i.e. 80.4% and 65.9% capacitance retentions at 1 and 4 A g⁻¹, respectively). These beneficial effects proved the synergistic contribution between p-rGO and Fe₃O₄. Hence, such ultrafine magnetite particles grown onto a porous reduced GO network directly imprinted onto a Ni substrate could be a promising candidate for high performance energy storage aims.

Here, we report a simple and green electrochemical route to deposition of Fe₃O₄ nanoparticle-porous reduced graphene oxide (p-rGO) nanocomposite onto nickel foam substrate, which is directly used as a binder-free charge storage electrode.     

Nanostructured diatom earth SiO2 negative electrodes with superior electrochemical performance for lithium ion batteries

Diatomaceous earth (DE) is a naturally occurring silica source constituted by fossilized remains of diatoms, a type of hard-shelled algae, which exhibits a complex hierarchically nanostructured porous silica network. In this work, we analyze the positive effects of reducing DE SiO₂ particles to the sub-micrometer level and implementing an optimized carbon coating treatment to obtain DE SiO₂ anodes with superior electrochemical performance for Li-ion batteries. Pristine DE with an average particle size of 17 μm is able to deliver a specific capacity of 575 mA h g⁻¹ after 100 cycles at a constant current of 100 mA g⁻¹, and reducing the particle size to 470 nm enhanced the reversible specific capacity to 740 mA h g⁻¹. Ball-milled DE particles were later subjected to a carbon coating treatment involving the thermal decomposition of a carbohydrate precursor at the surface of the particles. Coated ball-milled silica particles reached stable specific capacities of 840 mA h g⁻¹ after 100 cycles and displayed significantly improved rate capability, with discharge specific capacities increasing from 220 mA h g⁻¹ (uncoated ball-milled SiO₂) to 450 mA h g⁻¹ (carbon coated ball-milled SiO₂) at 2 A g⁻¹. In order to trigger SiO₂ reactivity towards lithium, all samples were subjected to an electrochemical activation procedure prior to electrochemical testing. XRD measurements on the activated electrodes revealed that the initial crystalline silica was completely converted to amorphous phases with short range ordering, therefore evidencing the effective role of the activation procedure.

Diatomaceous earth SiO₂ anodes with superior electrochemical performance are obtained by ball milling, carbon coating and electrochemical activation of SiO₂ particles.     

Self-assembled hierarchical porous NiMn2O4 microspheres as high performance Li-ion battery anodes

Hierarchical structured porous NiMn₂O₄ microspheres assembled with nanorods are synthesized through a simple hydrothermal method followed by calcination in air. As anode materials for lithium ion batteries (LIBs), the NiMn₂O₄ microspheres exhibit a high specific capacity. The initial discharge capacity is 1126 mA h g⁻¹. After 1000 cycles, the NiMn₂O₄ demonstrates a reversible capacity of 900 mA h g⁻¹ at a current density of 500 mA g⁻¹. In particular, the porous NiMn₂O₄ microspheres still could deliver a remarkable discharge capacity of 490 mA h g⁻¹ even at a high current density of 2 A g⁻¹, indicating their potential application in Li-ion batteries. This excellent electrochemical performance is ascribed to the unique hierarchical porous structure which can provide sufficient contact for the transfer of Li⁺ ion and area for the volume change of the electrolyte leading to enhanced Li⁺ mobility.

Hierarchical structured porous NiMn₂O₄ microspheres assembled with nanorods are synthesized through a simple hydrothermal method followed by calcination in air.     

Design of highly porous Fe3O4@reduced graphene oxide via a facile PMAA-induced assembly

Advances in the synthesis and processing of graphene-based materials have presented the opportunity to design novel lithium-ion battery (LIB) anode materials that can meet the power requirements of next-generation power devices. In this work, a poly(methacrylic acid) (PMAA)-induced self-assembly process was used to design super-mesoporous Fe₃O₄ and reduced-graphene-oxide (Fe₃O₄@RGO) anode materials. We demonstrate the relationship between the media pH and Fe₃O₄@RGO nanostructure, in terms of dispersion state of PMAA-stabilized Fe₃O₄@GO sheets at different surrounding pH values, and porosity of the resulted Fe₃O₄@RGO anode. The anode shows a high surface area of 338.8 m² g⁻¹ with a large amount of 10–40 nm mesopores, which facilitates the kinetics of Li-ions and electrons, and improves electrode durability. As a result, Fe₃O₄@RGO delivers high specific-charge capacities of 740 mA h g⁻¹ to 200 mA h g⁻¹ at various current densities of 0.5 A g⁻¹ to 10 A g⁻¹, and an excellent capacity-retention capability even after long-term charge–discharge cycles. The PMAA-induced assembly method addresses the issue of poor dispersion of Fe₃O₄-coated graphene materials—which is a major impediment in the synthesis process—and provides a facile synthetic pathway for depositing Fe₃O₄ and other metal oxide nanoparticles on highly porous RGO.

Advances in the synthesis and processing of graphene-based materials have presented the opportunity to design novel lithium-ion battery (LIB) anode materials that can meet the power requirements of next-generation power devices.     

A tin(iv) oxides/carbon nanotubes composite with core-tubule structure as an anode material for high electrochemistry performance LIBs

SnO₂/CNTs composites with core-tubule structure are prepared by a facile wet chemical method. The investigation of electrochemical characteristics of the SnO₂/CNTs composites shows that the composites exhibit some advantages, such as stable core-tubule structure, small particle size of SnO₂, low electron-transfer resistance and faster lithium ion migration speed. The final product synthesized under optimized conditions can release a stable capacity of about 743 mA h g⁻¹ after 100 cycles at the current density of 0.4 A g⁻¹, 598 mA h g⁻¹ after 500 cycles at the current density of 4 A g⁻¹. Even at a super high current density of 8 A g⁻¹, the composite can still deliver a steady capacity of 457 mA h g⁻¹, and the discharge capacity can be restored to 998 mA h g⁻¹ when current density is decreased to 0.4 A g⁻¹.

SnO₂/CNTs composites with core-tubule structure are prepared by a facile wet chemical method.     

Facile thermochemical conversion of FeOOH nanorods to ZnFe2O4 nanorods for high-rate lithium storage

We successfully prepared ZnFe₂O₄ nanorods (ZFO-NRs) by a simple thermochemical reaction of FeOOH nanorods with Zn(NO₃)₂ to use as an anode material in lithium-ion batteries. The FeOOH nanorod shape was well maintained after conversion into ZFO-NR with the formation of porous structures. The nanorod structure and porous morphology facilitate Li⁺ transport, improve the reaction rates owing to the larger contact area with the electrolyte, and reduce the mechanical stress during lithiation/delithiation. The ZFO-NR electrode exhibited a reversible capacity of 725 mA h g⁻¹ at 1 A g⁻¹ and maintained a capacity of 668 mA h g⁻¹ at 2 A g⁻¹; these capacities are much higher and more stable than those of ZFO nanoparticles prepared by a hydrothermal method (ZFO-HT) (216 and 117 mA h g⁻¹ at 1 and 2 A g⁻¹, respectively). Although ZFO-NRs exhibited high, stable capacities at moderate current densities for charging and discharging, the capacity rapidly decreased under fast charging/discharging conditions (>4 A g⁻¹). However, carbonized ZFO-NR (C/ZFO-NR) exhibited an improved reversible capacity and rate capability resulting from an increased conductivity compared with ZFO-NRs. The specific capacity of C/ZFO-NRs at 1 A g⁻¹ was 765 mA h g⁻¹; notably, a capacity of 680 mA h g⁻¹ was maintained at 6 A g⁻¹.

ZnFe₂O₄ nanorods were prepared by a simple thermochemical conversion of FeOOH nanorods, and exhibited an improved reversible capacity and rate capability.     

Off-stoichiometric Na3−3xV2+x(PO4)3/C nanocomposites as cathode materials for high-performance sodium-ion batteries prepared by high-energy ball milling

Na₃V₂(PO₄)₃ (NVP) is regarded as a promising cathode material for sustainable energy storage applications. Here we present an efficient method to synthesize off-stoichiometric Na_3−3xV_2+x(PO₄)₃/C (x = 0–0.10) nanocomposites with excellent high-rate and long-life performance for sodium-ion batteries by high-energy ball milling. It is found that Na_3−3xV_2+x(PO₄)₃/C nanocomposites with x = 0.05 (NVP-0.05) exhibit the most excellent performance. When cycled at a rate of 1C in the range of 2.3–3.9 V, the initial discharge capacity of NVP-0.05 is 112.4 mA h g⁻¹, which is about 96% of its theoretical value (117.6 mA h g⁻¹). Even at 20C, it still delivers a discharge capacity of 92.3 mA h g⁻¹ (79% of the theoretical capacity). The specific capacity of NVP-0.05 is as high as 100.7 mA h g⁻¹ after 500 cycles at 5C, which maintains 95% of its initial value (106 mA h g⁻¹). The significantly improved electrochemical performance of NVP-0.05 is attributed to the decrease of internal resistance and increase of the Na⁺ ion diffusion coefficient.

Na₃V₂(PO₄)₃ (NVP) is regarded as a promising cathode material for sustainable energy storage applications.     

A nanostructured SnO2/Ni/CNT composite as an anode for Li ion batteries

A SnO₂/Ni/CNT nanocomposite was synthesized using a simple one-step hydrothermal method followed by calcination. A structural study via XRD shows that the tetragonal rutile structure of SnO₂ is maintained. Further, X-ray photoelectron spectroscopy (XPS) and Raman studies confirm the existence of SnO₂ along with CNTs and Ni nanoparticles. The electrochemical performance was investigated via cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge–discharge measurements. The nanocomposite has been used as an anode material for lithium-ion batteries. The SnO₂/Ni/CNT nanocomposite exhibited an initial discharge capacity of 5312 mA h g⁻¹ and a corresponding charge capacity of 2267 mA h g⁻¹ during the first cycle at 50 mA g⁻¹. Pristine SnO₂ showed a discharge/charge capacity of 1445/636 mA h g⁻¹ during the first cycle at 50 mA g⁻¹. This clearly shows the effects of the optimum concentrations of CNTs and Ni. Further, the nanocomposite (SnNiCn) shows a discharge capacity as high as 919 mA h g⁻¹ after 210 cycles at a current density of 400 mA g⁻¹ in a Li-ion battery set-up. Thus, the obtained capacity from the nanocomposite is much higher compared to pristine SnO₂. The higher capacity in the nanoheterostructure is due to the well-dispersed nanosized Ni-decorated stabilized SnO₂ along with the CNTs, avoiding pulverization as a result of the volumetric change of the nanoparticles being minimized. The material accommodates huge volume expansion and avoids the agglomeration of nanoparticles during the lithiation and delithiation processes. The Ni nanoparticles can successfully inhibit Sn coarsening during cycling, resulting in the enhancement of stability during reversible conversion reactions. They ultimately enhance the capacity, giving stability to the nanocomposite and improving performance. Additionally, the material exhibits a lower Warburg coefficient and higher Li ion diffusion coefficient, which in turn accelerate the interfacial charge transfer process; this is also responsible for the enhanced stable electrochemical performance. A detailed mechanism is expressed and elaborated on to provide a better understanding of the enhanced electrochemical performance.

SnO₂/Ni/CNT nanocomposite approach has been demonstrated which confers shielding against volume expansion due to the use of optimum % of Ni & CNT exhibiting superior cycling stability and rate capabilities of the stable electrode structure.     

In situ approach of cementite nanoparticles encapsulated with nitrogen-doped graphitic shells as anode nanomaterials for Li-ion and Na-ion batteries

Novel Fe₃C nanoparticles encapsulated with nitrogen-doped graphitic shells were synthesized by floating catalytic pyrolysis. Due to the short synthesis time and controllable pyrolytic temperature, the diameters of Fe₃C core nanoparticles ranged from 5 to 15 nm (Fe₃C@NGS900 prepared at 900 °C) and the average thickness of N-doped graphitic shells was ∼1.2 nm, leading to their high electrochemical performance: specific capacity of 1300 mA h g⁻¹ at current density 0.2 A g⁻¹, outstanding rate capability of 939 mA h g⁻¹ at 3 A g⁻¹, improved initial coulombic efficiency (Fe₃C@NGS900: 72.1% vs. NGS900 (pure graphitic shells): 52%) for lithium ion batteries (LIBs), and impressive long-term cycle performance (1399 mA h g⁻¹ maintained at 3 A g⁻¹ after 500 cycles for LIBs; 214 mA h g⁻¹ maintained at 1 A g⁻¹ after 500 cycles for sodium ion batteries).

Novel Fe₃C nanoparticles encapsulated with nitrogen-doped graphitic shells were synthesized by floating catalytic pyrolysis.

Because of the fast development of portable electronic devices and hybrid electric vehicles, lithium ion batteries (LIBs)^1–8 and sodium ion batteries (NIBs)^9–16 with high energy/power density, good cycling performance, and lack of memory effects are in ever-increasing need. Due to the low theoretical capacity of carbon materials (graphite: 372 mA h g⁻¹),^8,17–22 optimizing the morphology of graphitic electrode materials has been important to improve specific capacity.^1–7,23,24 On the other hand, chemical doping (e.g., N, S, B, P) is an effective strategy to raise their specific capacity by increasing conductivity or active sites for Li⁺ or Na⁺ storage.^3,4,25–28 Additionally, metallic compounds (e.g., Fe₃C) have been also introduced into improving electrochemical performance of carbon anodes, because such materials are proposed to activate some components for reversible transformation of the solid electrolyte interface (SEI) and further benefit reversible capacity.^13,16,29,30 Unfortunately, most of them have been prepared by complex methods including tedious synthetic steps or long-time annealing,^6,17–19 from which, it is hard to prepare Fe₃C particles with desirable small sizes.^8,13,15,16 Thus, developing appropriate Fe₃C/C electrode materials still requires further research.In this work, Fe₃C nanoparticles encapsulated with nitrogen-doped graphitic shells (Fe₃C@NGS) were in situ approached from floating catalytic pyrolysis. Due to the short annealing time of the pyrolysis, Fe₃C@NGSs was prepared with controllable sizes. Furthermore, the in situ approach led the graphitic shells just grew on the surface of Fe₃C core nanoparticles, which improved electron transfer between the cores and the shells. Thus, such nanoparticles might be a superb electrode material towards high performance applications of LIBs and NIBs.For preparing the Fe₃C@NGSs with controllable sizes, floating catalytic pyrolysis was carried out to shorten synthetic time: the gas mixture was introduced into quartz pipe furnace, which was set at 700–1100 °C. For a typical experiment, nitrogen (flow rate: 80 L h⁻¹), acetylene (10 mL min⁻¹) and ammonia (100 mL min⁻¹) gases were embedded into iron pentacarbonyl held at 10 °C to form the gas mixture. After the pyrolysis, Fe₃C@NGS was collected at the other end of the quartz pipe. The details of materials characterization and electrochemical measurements can be found in ESI.^†As shown in transmission electron microscope (TEM) images of the prepared Fe₃C@NGSs (Fig. 1a–c and S1a of ESI^†), the metallic cores (dark section) are encapsulated with their shell (light section). The XRD results (Fig. 1d) shows the metallic core nanoparticles are Fe₃C. Thus, the sample prepared at 900 and 1100 °C have been marked with Fe₃C@NGS900 and Fe₃C@NGS1100, respectively. However, the sample prepared at 700 °C, which has been marked with FN@NGS700, has been oxidized in air at room temperature, because of its poor graphitic layers. Compared with XRD pattern of Fe₃C@NGS900, those peaks of Fe₃C@NGS1100 are much sharper indicating much bigger ferrous cores of Fe₃C@NGS1100. According to the TEM and high resolution TEM (HRTEM) images for Fe₃C@NGS900 (Fig. 1a–c, S1a, and b^†), the diameter of the core nanoparticles is ranged from 5–15 nm and the average thickness of their shells is ∼1.2 nm, respectively. Moreover, the spacing of the lattice fringes (Fig. S1a and b^†) is ∼0.34 nm corresponding to the characteristic (002) peak of graphite implying high graphitization of those shells.^25,28,31 In the HRTEM images, every Fe₃C cores is found to be a single crystal and encapsulated with the graphitic shell. The boundary between Fe₃C cores and N-doped graphitic shells is continuous and distinguished, indicating Fe₃C cores are tightly encapsulated with the graphitic shell.Open in a separate windowFig. 1(a) TEM images of core-shells nanoparticles (FN@NGS700) prepared at 700 °C, TEM and HRTEM (inset) images of nanoparticles (Fe₃C@NGS900) prepared at 900 °C (b) and nanoparticles (Fe₃C@NGS1100) prepared at 1100 °C (c) and (d) XRD patterns of core-shells nanoparticles prepared at different temperatures.Fe₃C@NGS samples have been also analyzed by X-ray photoelectron spectroscopy (XPS), which suggests Fe₃C@NGS samples contain Fe, C, O and N atoms (FN@NGS700: C content of 16.3 wt%, Fe content of 54.6 wt%, N content of 1.6 wt% and O content of 27.5 wt%; Fe₃C@NGS900: C content of 26 wt%, Fe content of 67 wt%, N content of 2 wt% and O content of 5 wt%; Fe₃C@NGS1100: C content of 33.7 wt%, Fe content of 52 wt%, N content of 2.3 wt% and O content of 7 wt%). According to XPS results of Fe₃C@NGS900, the weight ratio of Fe₃C cores and graphitic shells is 3.39 : 1. Additionally, the element contents of Fe₃C@NGS samples are different due to their morphology and structure (Fig. 1). Higher O content of FN@NGS700 is because its ferrous cores have been oxidized, and thick-walled graphitic shells (∼3 nm) of Fe₃C@NGS1100 leads to its higher C content, as XPS can only measure element contents of surface of samples.The electrochemical properties of the Fe₃C@NGS-based electrodes have been shown in Fig. 2 and ​and3.3. Since FN@NGS700 sample has been completely oxidized, electrochemical properties of ferrous oxide has not been measured. The cyclic voltammetry (CV) curves of Fe₃C@NGS900-based electrode show details of possible lithium storage process. Besides the similar peak (0.5 V) in the initial cycle for the SEI formation, two reduction peaks are located at 0.7 and ∼1.5 V, corresponding to the reduction of some SEI components (Li⁺ insertion). During the Li⁺ extraction process, the corresponding oxidation peaks is found to shift from ∼1.7 to ∼1.9 V. The almost overlapped oxidation peaks demonstrate good reversibility and cycling stability of core–shell Fe₃C@NGS.¹⁵ As shown in Fig. 2b, the galvanostatic discharge–charge (GDC) profiles of Fe₃C@NGS900 in a voltage range of 0.005–3 V (vs. Li⁺/Li) exhibits the typical shape of Fe₃C@NGS-based anodes, and Fe₃C@NGS900 delivers initial charge and discharge capacities of 1246.7 and 1729.1 mA h g⁻¹, respectively. The initial columbic efficiency (CE) reaches up to 72.1% which is higher than 52% of pure graphitic shells.³ For Fe₃C@NGSs electrode, the phenomenon of capacity increment is related to growing reversibly SEI film via the decomposition of electrolyte due to the catalysis of Fe₃C.^16,29 From the second cycle, the shape of the discharge profiles changes with respect to that of the first cycle, which may be due to the modification of the SEI film.¹²Open in a separate windowFig. 2Electrochemical performance of prepared nanoparticles as anodes for LIBs: CV profile (a) of Fe₃C@NGS900 at a scan rate of 0.1 mV s⁻¹ between 0.01–3 V vs. Li⁺/Li for the 1st–2nd charge/discharge cycles; (b) the galvanostatic charge–discharge profiles of Fe₃C@NGS900; (c) charge–discharge cycling performance of Fe₃C@NGS core–shell nanoparticles at different current densities from 0.2 to 3 A g⁻¹ at room temperature; (d) cycling performance of Fe₃C@NGS at 3 A g⁻¹.Open in a separate windowFig. 3Electrochemical performance of the prepared nanoparticles as anodes for NIBs: (a) CV profile of Fe₃C@NGS900 at a scan rate of 0.1 mV s⁻¹ between 0.01–2.5 V vs. Na⁺/Na for the 1st–3rd charge/discharge cycles. (b) The galvanostatic charge–discharge profiles of Fe₃C@NGS900; (c) cycling performance of Fe₃C@NGS at a current density of 1 A g⁻¹, and the corresponding columbic efficiency.The excellent rate capability of Fe₃C@NGS-based anodes have been investigated by testing charge/discharge at current densities of 0.2, 0.5, 1 and 3 A g⁻¹ for every 5 cycles (Fig. 2c). At the corresponding rates, the reversible capacities are 1300, 1101, 1062 and 939 mA h g⁻¹ for Fe₃C@NGS900; 925, 721, 690 and 663 mA h g⁻¹ for Fe₃C@NGS1100, indicating the smaller size of the Fe₃C nanoparticles might enhance their electrochemical capability. Compared with reported pure graphitic shells (NGS900 prepared by removing Fe₃C cores of Fe₃C@NGS900),³ the core–shell nanoparticles (Fe₃C@NGS900) exhibits impressive rate performances (NGS900: 760 mA h g⁻¹ at 0.5 A g⁻¹, 620 mA h g⁻¹ at 1 A g⁻¹ and 340 mA h g⁻¹ at 5 A g⁻¹), which might be caused by Fe₃C, as a good conductor of electricity, can effectively improve electrical conductivity of carbon electrode material.¹³ Calculated from eqn (1) of ESI,^† specific capacity of Fe₃C cores of Fe₃C@NGS900 can be evaluated (1199 mA h g⁻¹ at 0.5 A g⁻¹ and 1193 mA h g⁻¹ at 1 A g⁻¹, respectively). Based on the conversion mechanism for lithium storage, if possible, Fe₃C can store only 1/6 Li per unit (∼26 mA h g⁻¹),¹⁵ which is negligible regarding to the high capacity of ∼1300 mA h g⁻¹. The specific capacity of Fe₃C is larger than what it should be, which might be resulted from the pseudocapacity on the interface between the material and the electrolyte.¹⁶ For evaluating N doping structure in the graphitic shells, Fe₃C@NGS samples have been prepared with different percent of doping content at 900 °C by introducing ammonia with different flow rates (0, 30, 100 or 500 mL min⁻¹). As a result, Fe₃C@GS900 prepared without ammonia has graphitic shells without N-doping leading to its poor electrochemical performance: at current densities of 0.2, 0.5, 1 and 3 A g⁻¹, its reversible capacities are 575, 492, 458 and 402 mA h g⁻¹ (Fig. S4^†); the performances of Fe₃C@NGS900 (ammonia flow rate: 100 mL min⁻¹; content of N: 2 wt%) and Fe₃C@NGS900A (ammonia flow rate: 30 mL min⁻¹; content of N: 1.5 wt%) are similar; the sample prepared with ammonia flow rates of 500 mL min⁻¹ (FN@NGS900B) was violent oxidized to ferrous oxide in the air. Hence, N doping structure in graphitic shells has been confirmed to enhance diffusion.The long-term cycling performance of these two electrodes also has been investigated in Fig. 2d. The Fe₃C@NGS-based anode exhibits a favorable reversible capacity, which can reach 1399 mA h g⁻¹ after 500 discharge/charge cycles at 3.0 A g⁻¹, showing high capacity retention with CE of ∼100%. The capacities of Fe₃C samples increases with cycle number rising (from 120 to 450), which might be attributed to the pseducapacity presented by the Fe₃C.¹⁶To better study the kinetic properties of Fe₃C@NGS900 and Fe₃C@NGS1100, Fig. S3 of ESI^† shows the Nyquist plots and equivalent circuit obtained from electrochemical impedance spectroscopy (EIS) measurements. Here, R_s represents the ohmic resistance of the battery. Constant phase element (CPE) represents the double layer capacitive reactance between the electrode materials and the electrolyte. The semicircles and straight lines correspond to the electrochemical polarization impedance (R_p) and Warburg resistance (W), respectively.³² Both the fitted R_s value (5.526 Ω) and R_p value (20.826 Ω) for Fe₃C@NGS900 electrode is much lower than that Fe₃C@NGS1100 electrode (R_s: 8.501 Ω; R_p: 43.665 Ω), indicating the superior redox kinetics in the Fe₃C@NGS900 composite.In order to study the electrochemical properties of the Fe₃C@NGSs electrode as anodes for NIBs, CV analysis has been carried out at a scanning rate of 0.1 mV s⁻¹ between 0.005 and 2.5 V vs. Na⁺/Na. As shown in Fig. 3a, there are two irreversible reduction peaks around 2.02 V and 0.83 V found during the initial cathodic scan, which could be ascribed to the interaction of Na ions with specific functional groups and the decomposition of electrolyte along with the formation of SEI film on the electrode surfaces.^33,34 For the subsequent cycles, the peak at 2.02 V disappears and the peak at 0.83 V shifts to 0.60 V. For the anodic scan, the main oxidation peak ranging from about 0.47 V to 1.28 V is supposed to be the desodiation reactions.³⁵Fig. 3b depicts the GDC curves of the Fe₃C@NGS900-based electrode for the 1st–5th cycle at current density of 0.1 A g⁻¹. The large irreversible capacity in the 1st cycle is attributed to the SEI formation and the irreversible insertion of sodium ion with a relatively large ionic radius.^22,23 Following the first cycle, the charge–discharge curves become more linear which exhibits a higher and more stable CE indicating a stable SEI layer formed in the first cycle.Meanwhile, cycling performance of Fe₃C@NGS-based anodes for NIBs have been shown in Fig. 3c. In the extended cycling test at 1 A g⁻¹, a reversible capacity 214 mA h g⁻¹ of Fe₃C@NGS900 electrode is still maintained after 500 cycles, which is ∼2 times the capacity delivered by the Fe₃C@NGS1100 indicating excellent cycling stability of Fe₃C@NGS900. Thus, Fe₃C@NGS-base anodes holds great potential as a promising candidate compared with other carbonaceous anode materials for NIBs (113 mA h g⁻¹ at 1 A g⁻¹ (modified PFR/C),³⁶ 188.6 mA h g⁻¹ at 0.1 A g⁻¹ after 300 cycles (S/C),³³ 150 mA h g⁻¹ at 1 A g⁻¹ after 200 cycles (S/graphene)³⁵).It is noticed that the excellent electrochemical performance of the prepared Fe₃C@NGSs is apparently ascribed to their novel structure. First, because of in situ growing graphitic shells on the surface of Fe₃C during floating catalytic pyrolysis, contact between Fe₃C cores and graphitic shells effectively increases, leading to lower contact resistance and faster electron transfer between the cores and the shells, comparing with traditional ferrous/carbon composites generated by multistep carbonization approach.^8,13,15,16 Second, due to floating catalytic pyrolysis, sizes of Fe₃C core nanoparticles have been under control (Fe₃C@NGS900: 5–15 nm vs. Fe₃C@NGS1100: 15–40 nm). Smaller size of Fe₃C core nanoparticles might increase surface of Fe₃C nanoparticles, leading to distinguished improvement of their electrochemical performance (Fig. 2 and ​and3),3), due to active sites for Li⁺ or Na⁺ storage rising. For comparison, sizes of reported Fe₃C composites have been listed: Fe₃C@C: 60 nm nanoparticles encapsulated with 4 nm carbon shells;⁸ Fe₃C@PC: 29 ± 5 nm nanoparticles embedded in 300 nm porous carbon;¹⁶ Fe₃C/C: 300 nm;¹³ Fe@Fe₃C/C: 28–58 nm Fe@Fe₃C nanoparticles.¹⁵ Third, introducing Fe₃C to electrode material can promote the reversible formation/decomposition of the SEI film, causing improvement of initial CE (72.1% vs. 52%) for LIBs, due to the catalysis function of Fe₃C.^16,29 Fourth, due to in situ N-doping (∼2 wt%) during their floating catalytic pyrolysis, such defects of the graphitic shells might offer lots channels for fast diffusion of electrolyte and Li⁺/Na⁺ into those nanoparticles. Fifth, such prepared core–shell nanoparticles have ultra thin-walled graphitic shells (∼1.2 nm shown in Fig. 1d), which shorten the diffusion route of ions and electrolyte. All above confirm Fe₃C@NGS900 has novel structure towards the electrochemical applications, compared with the reported works: Fe@Fe₃C/C sample was prepared by sol–gel and carbonization approach, and whether its pure Fe cores was good for Li⁺ storage was doubtful;¹⁵ the Fe₃C@C nanoparticles were prepared with no doping carbon shells;⁸ the graphitization of carbon structure was doubtful, when ferrous/carbon composites were prepared by polymerization-carbonization of iron phthalocyanine¹³ or hydrothermal method-carbonization.¹⁶ Thus, the Fe₃C@NGS900 performs better (1300 mA h g⁻¹ at current density of 0.2 A g⁻¹; 939 mA h g⁻¹ at 3 A g⁻¹) than many reported ferrous/carbon composite anode materials for LIBs (0.2 A g⁻¹: <382 (Fe@Fe₃C/C),¹⁵ ∼480 (Fe₃C/C),⁸ 787.9 (Fe₂O₃/C),⁵ ∼850 (Fe₃O₄/C),¹⁹ 873 (N,S/C),³⁶ and 881 (Fe₃O₄/C)⁹ mA h g⁻¹; 3 A g⁻¹: ∼300 (Fe₂O₃@C),¹⁸ ∼370 (FeS@C)²¹ and ∼612 (Fe₂O₃/C)¹⁴ mA h g⁻¹).     

Substantially enhanced rate capability of lithium storage in Na2Ti6O13 with self-doping and carbon-coating

Na₂Ti₆O₁₃ (NTO) has recently been reported for lithium ion storage and showed very promising results. In this work, we report substantially enhanced rate capability in NTO nanowires by Ti(iii) self-doping and carbon-coating. Ti(iii) doping and carbon coating were found to work in synergy to increase the electrochemical performances of the material. For 300 cycles at 1C (1C = 200 mA g⁻¹) the charge capacity of the electrode is 206 mA h g⁻¹, much higher than that (89 mA h g⁻¹) of the pristine NTO electrode. For 500 cycles at 5C the electrode can still deliver a charge capacity of 180.5 mA h g⁻¹ with a high coulombic efficiency of 99%. At 20C the capacity of the electrode is 2.6 times that of the pristine NTO. These results clearly demonstrate that the Ti(iii) self-doping and uniform carbon coating significantly enhanced the kinetic processes in the NTO nanowire crystal, making it possible for fast charge and discharge in Li-ion batteries.

Ti³⁺ self-doping and carbon-coating are efficient approaches to simultaneously improve the rate capability and cyclability of Na₂Ti₆O₁₃ nanowires for lithium storage.     

Rapid fabrication of porous silicon/carbon microtube composites as anode materials for lithium-ion batteries

Herein, we present a simple and rapid method to synthesize porous silicon/carbon microtube composites (PoSi/CMTs) by adopting a unique configuration of acid etching solution. The CMTs can act as both conductive agent and buffer for Si volume change during the charge and discharge process. The highly reversible capacity and excellent rate capability can be ascribed to the structure, where porous silicon powders are wrapped by a network of interwoven carbon microtubes. The composites show specific capacities of more than 1712 mA h g⁻¹ at a current density of 100 mA g⁻¹, 1566 mA h g⁻¹ at 200 mA g⁻¹, 1407 mA h g⁻¹ at 400 mA g⁻¹, 1177 mA h g⁻¹ at 800 mA g⁻¹, 1107 mA h g⁻¹ at 1000 mA g⁻¹, 798 mA hg⁻¹ at 2000 mA g⁻¹, and 581 mA h g⁻¹ at 3000 mA g⁻¹ and maintain a value of 1127 mA h g⁻¹ after 100 cycles at a current density of 200 mA g⁻¹. Electrochemical impedance spectroscopy (EIS) measurements prove that charge transfer resistance of PoSi/CMT composites is smaller than that of pure PoSi. In this study, we propose a quick, economical and feasible method to prepare silicon-based anode materials for lithium-ion batteries.

We added additives to the acid etching solution and prepared the silicon/carbon microtubes composites using a simple and fast method.     

Zn2+ removal from the aqueous environment using a polydopamine/hydroxyapatite/Fe3O4 magnetic composite under ultrasonic waves

In this study, an easily magnetically recoverable polydopamine (PDA)-modified hydroxyapatite (HAp)/Fe₃O₄ magnetic composite (HAp/Fe₃O₄/PDA) was suitably synthesized to exploit its adsorption capacity to remove Zn²⁺ from aqueous solution, and its structural properties were thoroughly examined using different analytical techniques. The effect of multiple parameters like pH, ultrasonic power, ultrasonic time, adsorbent dose, and initial Zn²⁺ concentration on the adsorption efficiency was assessed using RSM-CCD. According to the acquired results, by increasing the adsorbent quantity, ultrasonic power, ultrasonic time, and pH, the Zn²⁺ adsorption efficiency increased and the interaction between the variables of ultrasonic power/Zn²⁺ concentration, pH/Zn²⁺ concentration, pH/absorbent dose, and ultrasonic time/adsorbent dose has a vital role in the Zn²⁺ adsorption. The uptake process of Zn²⁺ onto PDA/HAp/Fe₃O₄ followed Freundlich and pseudo-second order kinetic models. The maximum capacity of Zn²⁺ adsorption (q_m) obtained by PDA/HAp/Fe₃O₄, HAp/Fe₃O₄, and HAp was determined as 46.37 mg g⁻¹, 40.07 mg g⁻¹, and 37.57 mg g⁻¹, respectively. Due to its good performance and recoverability (ten times), the HAp/Fe₃O₄/PDA magnetic composite can be proposed as a good candidate to eliminate Zn²⁺ ions from a water solution.

A magnetically recoverable polydopamine (PDA)-modified hydroxyapatite (HAp)/Fe₃O₄ magnetic composite (HAp/Fe₃O₄/PDA) was synthesized to exploit its adsorption capacity to remove Zn²⁺ from aqueous solution and the structural properties were examined.