Nitrogen-doped hierarchically porous carbon networks: synthesis and applications in lithium-ion battery, sodium-ion battery and zinc-air battery

Preparation of Hollow Core-Shell Fe3O4/Nitrogen-Doped Carbon Nanocomposites for Lithium-Ion Batteries

Iron oxides are potential electrode materials for lithium-ion batteries because of their high theoretical capacities, low cost, rich resources, and their non-polluting properties. However, iron oxides demonstrate large volume expansion during the lithium intercalation process, resulting in the electrode material being crushed, which always results in poor cycle performance. In this paper, to solve the above problem, iron oxide/carbon nanocomposites with a hollow core-shell structure were designed. Firstly, an Fe2O3@polydopamine nanocomposite was prepared using an Fe2O3 nanocube and dopamine hydrochloride as precursors. Secondly, an Fe3O4@N-doped C composite was obtained by means of further carbonization treatment. Finally, Fe3O4@void@N-Doped C-x composites with core-shell structures with different void sizes were obtained by means of Fe3O4 etching. The effect of the etching time on the void size was studied. The electrochemical properties of the composites when used as lithium-ion battery materials were studied in more detail. The results showed that the sample that was obtained via etching for 5 h using 2 mol L-1 HCl solution at 30 °C demonstrated better electrochemical performance. The discharge capacity of the Fe3O4@void@N-Doped C-5 was able to reach up to 1222 mA g h-1 under 200 mA g-1 after 100 cycles.

MOF-derived Ru@ZIF-8 catalyst with the extremely low metal Ru loading for selective hydrogenolysis of C–O bonds in lignin model compounds under mild conditions

A MOF-derived Ru@ZIF-8 catalyst with extremely low Ru loading effectively cleaved the C–O bonds of lignin model compounds under mild conditions.

Enhanced stability of nitrogen doped porous carbon fiber on cathode materials for high performance lithium–sulfur batteries

Lithium–sulfur (Li–S) batteries are considered to be one of the candidates for high-energy density storage systems due to their ultra-high theoretical specific capacity of 1675 mA h g⁻¹. However, problems of rapid capacity decay, sharp expansion in volume of the active material, and the shuttle effect have severely restricted their subsequent development and utilization. Herein, we design a nitrogen-doped porous carbon nanofiber (NPCNF) network as a sulfur host by the template method. The NPCNF shows a feather-like structure. After loading sulfur, the NPCNF/S composite can maintain a hierarchically porous structure. A high discharge capacity of 1301 mA h g⁻¹ is delivered for the NPCNT/S composite at 0.1C. The reversible charge/discharge capacity at 2C is 576 mA h g⁻¹, and 700 mA h g⁻¹ is maintained after 500 cycles at 0.5C. The high electrochemical performance of this NPCNT/S composite is attributed to the synergy effects of abundant N active sites and high electrical conductivity of the material.

The conductive network of nitrogen-doped porous carbon nanofibers was successfully prepared by the template method. The doping of nitrogen and the synergistic effect of mesopores and micropores reduce the energy barrier of Li⁺ migration in the material.

Recent advances in the template-confined synthesis of two-dimensional materials for aqueous energy storage devices

The template-confined synthesis strategy is a simple and effective methodology to prepare two-dimensional nanomaterials. It has multiple advantages including green process, controllable morphology and adjustable crystal structure, and therefore, it is promising in the energy storage realm to synthesize high-performance electrode materials. In this review, we summarize the recent advances in the template-confined synthesis of two-dimensional nanostructures for aqueous energy storage applications. The material design is discussed in detail to accommodate target usage in aqueous supercapacitors and zinc metal batteries. The remaining challenges and future prospective are also covered.

Self-Templating Synthesis of Hollow Co3O4 Nanoparticles Embedded in N,S-Dual-Doped Reduced Graphene Oxide for Lithium Ion Batteries

The design and synthesis of hollow-nanostructured transition metal oxide-based anodes is of great importance for long-term operation of lithium ion batteries. Herein, we report a two-step calcination strategy to fabricate hollow Co₃O₄ nanoparticles embedded in a N,S-co-doped reduced graphene oxide framework. In the first step, core-shell-like Co@Co₃O₄ embedded in N,S-co-doped reduced graphene oxide is synthesized by pyrolysis of a Co-based metal organic framework/graphene oxide precursor in an inert atmosphere at 800 °C. The designed hollow Co₃O₄ nanoparticles with an average particle size of 25 nm and wall thickness of about 4-5 nm are formed by a further calcination process in air at 250 °C via the nanoscale Kirkendall effect. Both micropores and mesopores are generated in the HoCo₃O₄/NS-RGO framework. Benefiting from the hierarchical porous structure of the hollow Co₃O₄ and the co-doping of nitrogen and sulfur atoms in reduced graphene oxide, the thus-assembled battery exhibits a high specific capacity of 1590 mAh g^-1 after 600 charge-discharge cycles at 1 A g^-1 and a promising rate performance from 0.2 to 10 A g^-1.

Improving the Performance of Zn-Air Batteries with N-Doped Electroexfoliated Graphene

The constantly growing demand for active, durable, and low-cost electrocatalysts usable in energy storage devices, such as supercapacitors or electrodes in metal-air batteries, has triggered the rapid development of heteroatom-doped carbon materials, which would, among other things, exhibit high catalytic activity in the oxygen reduction reaction (ORR). In this article, a method of synthesizing nitrogen-doped graphene is proposed. Few-layered graphene sheets (FL-graphene) were prepared by electrochemical exfoliation of commercial graphite in a Na2SO4 electrolyte with added calcium carbonate as a separator of newly-exfoliated FL-graphene sheets. Exfoliated FL-graphene was impregnated with a suspension of green algae used as a nitrogen carrier. Impregnated FL-graphene was carbonized at a high temperature under the flow of nitrogen. The N-doped FL-graphene was characterized through instrumental methods: high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. Electrochemical performance was determined using cyclic voltamperometry and linear sweep voltamperometry to check catalytic activity in ORR. The N-doped electroexfoliated FL-graphene obeyed the four-electron transfer pathways, leading us to further test these materials as electrode components in rechargeable zinc-air batteries. The obtained results for Zn-air batteries are very important for future development of industry, because the proposed graphene electrode materials do not contain any heavy and noble metals in their composition.

On the Possibility of Helium Adsorption in Nitrogen Doped Graphitic Materials

The potassium salt of polyheptazine imide (K-PHI) is a promising photocatalyst for various chemical reactions. From powder X-ray diffraction data an idealized structural model of K-PHI has been derived. Using atomic coordinates of this model we defined an energetically optimized K-PHI structure, in which the K ions are present in the pore and between the PHI-planes. The distance between the anion framework and K+ resembles a frustrated Lewis pair-like structure, which we denote as frustrated Coulomb pair that results in an interesting adsorption environment for otherwise non-adsorbing, non-polar gas molecules. We demonstrate that even helium (He) gas molecules, which are known to have the lowest boiling point and the lowest intermolecular interactions, can be adsorbed in this polarized environment with an adsorption energy of  - 4.6 kJ mol-1 per He atom. The interaction between He atoms and K-PHI is partially originating from charge transfer, as disclosed by our energy decomposition analysis based on absolutely localized molecular orbitals. Due to very small charge transfer interactions, He gas adsorption saturates at 8 at%, which however can be subject to further improvement by cation variation.

Recent Progress in Two-Dimensional Layered Double Hydroxides and Their Derivatives for Supercapacitors

High-performance supercapacitors have attracted great attention due to their high power, fast charging/discharging, long lifetime, and high safety. However, the generally low energy density and overall device performance of supercapacitors limit their applications. In recent years, the design of rational electrode materials has proven to be an effective pathway to improve the capacitive performances of supercapacitors. Layered double hydroxides (LDHs), have shown great potential in new-generation supercapacitors, due to their unique two-dimensional layered structures with a high surface area and tunable composition of the host layers and intercalation species. Herein, recent progress in LDH-based, LDH-derived, and composite-type electrode materials targeted for applications in supercapacitors, by tuning the chemical/metal composition, growth morphology, architectures, and device integration, is reviewed. The complicated relationships between the composition, morphology, structure, and capacitive performance are presented. A brief projection is given for the challenges and perspectives of LDHs for energy research.

Synthesis of a Porous C3N-Derived Framework with High Yield by Gallic Acid Cross-Linking Using Salt Melts

Porous carbon/nitrogen frameworks are an emerging class of noble organic materials with a wide range of potential applications. However, the design and controlled synthesis of those materials are still a challenge. Herein, we present the rational design of such a system with high microporosity, specific surface areas of up to 946 m² g^-1, and excellent condensation yields. The obtained noble frameworks were used for the delivery of larger organic molecules and changed the melting behavior of some daily drug molecules along their highly polarizable surfaces.

Multi-heteroatom-doped dual carbon-confined Fe3O4 nanospheres as high-capacity and long-life anode materials for lithium/sodium ion batteries

The lithium/sodium-ion storage properties of transition metal oxides often undergo startling volume variation and poor electrical conductivity. Herein, N, P and S doped dual carbon-confined Fe₃O₄ nanospheres (Fe₃O₄@C@G) are prepared by the multi-heteroatom-doped dual carbon-confined strategy. The first carbon layer results from multi-heteroatom-containing polymer derived N, P and S doped carbon to form Fe₃O₄@doped carbon core-shell nanostructure. And the second carbon layer results from the further encapsulated reduced graphene oxide (rGO) to form Fe₃O₄@doped carbon@graphene 3D architecture (Fe₃O₄@C@G). As expected, the resulting Fe₃O₄@C@G can be served as the universal anode materials towards lithium/sodium-ion batteries (LIBs/SIBs). Interestingly, Fe₃O₄@C@G delivers higher reversible capacity of 919 mAh g^-1 at 0.1 A g^-1 for LIBs. As for SIBs, Fe₃O₄@C@G also shows a high reversible capacity of 180 mAh g^-1 after 600 cycles at 0.1 A g^-1. Furthermore, the electrochemical reaction kinetics in LIBs/SIBs are investigated and Li⁺ full cells are also assembled to demonstrate its practical application.

Recent progress in pyrolyzed carbon materials as electrocatalysts for the oxygen reduction reaction

This review reports some recent advances in pyrolytic carbon as an ORR catalyst and explores its structure–activity relationship.

A Flexible and Boron-Doped Carbon Nanotube Film for High-Performance Li Storage

Boron-doped carbon nanotubes are a promising candidate for Li storage due to the unique electronic structure and high crystallinity brought by the boron dopants. However, the relatively low Li storage capacity has limited its application in the electrochemical energy storage field, which is mainly caused by the predominantly intact graphitic structure on their surface with limited access points for Li ion entering. Herein, we report a novel B-doped CNTs (py-B-CNTs) film, in which the CNTs possess intrinsically rough surface but flat internal graphitic structure. When used as a flexible anode material for LIBs, this py-B-CNTs film delivers significantly enhanced capacity than the conventional B-doped CNTs or the pristine CNTs films, with good rate capability and excellent cycling performance as well. Moreover, this flexible film also possesses excellent mechanical flexibility, making it capable of being used in a prototype flexible LIB with stable power output upon various bending states.

Integrated N-Co/Carbon Nanofiber Cathode for Highly Efficient Zinc-Air Batteries

In order to reduce the charge-transfer resistance, ohmic resistance, and ionic and electronic resistances arising from the polymer binder, designing and constructing self-standing and binder-free porous electrodes are very significant for energy storage and conversion devices. Herein, self-standing and binder-free porous N-Co carbon nanofiber (N-Co/CNF) cathodes are prepared for zinc-air batteries (ZABs) by an in situ electrospinning/plasma-etching method. The morphology and activity of the prepared electrodes are investigated by several characterization techniques. The prepared specimens exhibit a multilayered CNF structure, and a new CoN compound is produced after plasma-etching treatment. The N-Co/CNF-300-10 cathode demonstrates excellent electrocatalytic performance toward oxygen reduction reaction, with an onset potential and a half-wave potential of 0.995 and 0.853 V (vs reversible hydrogen electrode), respectively, which is comparable to that of 20% Pt/C. The N-Co/CNF-300-10 cathode acting as a self-standing electrode for ZABs exhibits a maximum discharge power density as high as 229 mW cm^-2 and a specific capacity of 659.6 mA h g_Zn^-1, which are much higher than those of the commercial catalysts, benefiting from the self-standing porous structure, N-doping, and more defects and active sites induced by plasma-etching. It provides an effective way to construct a self-standing porous electrode with controllable compositions for rechargeable metal-air batteries.

Synthesis and Electrochemical Performance of ZnSe Electrospinning Nanofibers as an Anode Material for Lithium Ion and Sodium Ion Batteries

ZnSe nitrogen-doped carbon composite nanofibers (ZnSe@N-CNFs) were derived as anode materials from selenization of electrospinning nanofibers. Electron microscopy shows that ZnSe nanoparticles are distributed in electrospinning nanofibers after selenization. Electrochemistry tests were carried out and the results show the one-dimensional carbon composite nanofibers reveal a great structural stability and electrochemistry performance by the enhanced synergistic effect with ZnSe. Even at a current density of 2 A g-1, the as-prepared electrodes can still reach up to 701.7 mA h g-1 after 600 cycles in lithium-ion batteries and 368.9 mA h g-1 after 200 cycles in sodium-ion batteries, respectively. ZnSe@N-CNFs with long cycle life and high capacity at high current density implies its promising future for the next generation application of energy storage.

Atomic Modulation and Structure Design of Carbons for Bifunctional Electrocatalysis in Metal-Air Batteries

With the extensive research and development of renewable energy technologies, there is an increasing interest in developing metal-free carbons as a new class of bifunctional electrocatalysts for boosting the performance of metal-air batteries. Along with significant understanding of the electrocatalytic nature and the rapid development of techniques, the activities of carbon electrocatalysts are well-tailored by introducing particular dopants/defects and structure regulation. Herein, the recent advances regarding the rational design of carbon-based electrocatalysts for the oxygen reduction reaction and oxygen evolution reaction are summarized, with a special focus on the bifunctional applications in Zn-air and Li-air batteries. Specifically, the atomic modulation strategies to regulate the electrocatalytic activities of carbons and structure modification are summarized to gain deep insights into bifunctional mechanisms and boost advanced Zn-air and Li-air batteries. The current challenges and future perspectives are also addressed to accelerate the exploration of promising bifunctional carbon catalysts for renewable energy technologies, particularly metal-air batteries.

An Interpenetrating Porous Organic Polymer as a Precursor for FeP/Fe2 P-Embedded Porous Carbon toward a pH-Universal ORR Catalyst

Interpenetrating porous organic polymers (PNFc-POP) inspired by the structure of DNA were synthesized through a two-stage polymerization method under catalyst-free conditions. A ferrocene-rich hyper-crosslinked polymer (Fc-melamine) was interwoven with cyclotriphosphazene-based conjugated porous polymer (PN-CMP) to obtain an interconnected polymer network (PNFc-POP). The sequential interpenetrating polymer network contained a diverse range of heteroatoms (P, N, O and Fe) and exhibited a large BET surface area. Simple pyrolysis of the dual polymer interweaved skeletons at 900 °C afforded nanocrystalline FeP/Fe₂ P-embedded N and P codoped porous carbon composites. The optimal catalyst obtained by the pyrolysis of PNFc-POP at 900 °C (PNFc-900) exhibited hierarchical porosity and large BET surface areas. It also exhibited excellent oxygen reduction reaction catalytic activities over the entire pH range. The onset potential (E_onset =1.01 V) and half-wave potential (E_1/2 =0.86 V) of PNFc-900 exceeded those of commercial Pt/C (E_onset =0.99 V and E_1/2 =0.84 V) in alkaline conditions. The obtained catalysts with a four-electron transfer pathway for the reduction of oxygen also displayed excellent long-term stability and methanol tolerance.

Urchin-like hierarchical H-Nb2O5 microspheres: synthesis, formation mechanism and their applications in lithium ion batteries

Urchin-like hierarchical Nb₂O₅ microspheres are successfully synthesized through a facile solvothermal method in glycerol-isopropanol mixed media followed by thermal treatment. The sample is characterized by XRD, FESEM, TEM, HRTEM, BET, and XPS, and the results reveal that the as-formed Nb₂O₅ microspheres have a pseudohexagonal structure and are composed of nanorods with an average diameter of ca. 20 nm. It is found that glycerol not only serves as a solvent but also acts as a reactant; furthermore, isopropanol plays an important part in the morphologies of the products. When used as anodic materials for lithium ion batteries, the Nb₂O₅ microspheres deliver initial discharge capacities of 201.7, 159.7, 148.5, 123.7, and 98.5 mA h g^-1 at the current densities of 0.5, 1, 2, 5, and 10C, respectively. Additionally, the discharge capacity of Nb₂O₅ remains at 105.5 mA h g^-1 even after 500 cycles at a high rate of 5C. The good electrochemical properties of the products may be ascribed to their large surface areas and hierarchical structures.

Nitrogen, Fluorine, and Boron Ternary Doped Carbon Fibers as Cathode Electrocatalysts for Zinc-Air Batteries

Zinc-air batteries with high-density energy are promising energy storage devices for the next generation of energy storage technologies. However, the battery performance is highly dependent on the efficiency of oxygen electrocatalyst in the air electrode. Herein, the N, F, and B ternary doped carbon fibers (TD-CFs) are prepared and exhibited higher catalytic properties via the efficient 4e^- transfer mechanism for oxygen reduction in comparison with the single nitrogen doped CFs. More importantly, the primary and rechargeable Zn-air batteries using TD-CFs as air-cathode catalysts are constructed. When compared to batteries with Pt/C + RuO₂ and Vulcan XC-72 carbon black catalysts, the TD-CFs catalyzed batteries exhibit remarkable battery reversibility and stability over long charging/discharging cycles.

The Concept of "Noble, Heteroatom-Doped Carbons," Their Directed Synthesis by Electronic Band Control of Carbonization, and Applications in Catalysis and Energy Materials

Carbonization of organic compounds with a highest occupied molecular orbital (HOMO) level more positive than 1.3 V practically automatically results in highly sp² -conjugated, heteroatom-doped carbons. Due to the stability of the starting compounds, carbon bond formation is restricted to result in morphologies with a surprisingly high local order which as such are noble, i.e., they are hard to oxidize and combust. The work function of electrons in these systems is so positive that the systems usually accept electrons, i.e., they oxidize other matter rather than being oxidized. Such noble, heteroatom-doped carbons have been proven to be efficient, metal-free electrocatalysts, but can be also beneficially used in the manufacturing of carbon nanomaterials for energy applications or as highly active, non-innocent catalytic supports.

Hollow TiO2@Co9S8 Core-Branch Arrays as Bifunctional Electrocatalysts for Efficient Oxygen/Hydrogen Production

Designing ever more efficient and cost-effective bifunctional electrocatalysts for oxygen/hydrogen evolution reactions (OER/HER) is greatly vital and challenging. Here, a new type of binder-free hollow TiO2@Co9S8 core-branch arrays is developed as highly active OER and HER electrocatalysts for stable overall water splitting. Hollow core-branch arrays of TiO2@Co9S8 are readily realized by the rational combination of crosslinked Co9S8 nanoflakes on TiO2 core via a facile and powerful sulfurization strategy. Arising from larger active surface area, richer/shorter transfer channels for ions/electrons, and reinforced structural stability, the as-obtained TiO2@Co9S8 core-branch arrays show noticeable exceptional electrocatalytic performance, with low overpotentials of 240 and 139 mV at 10 mA cm-2 as well as low Tafel slopes of 55 and 65 mV Dec-1 for OER and HER in alkaline medium, respectively. Impressively, the electrolysis cell based on the TiO2@Co9S8 arrays as both cathode and anode exhibits a remarkably low water splitting voltage of 1.56 V at 10 mA cm-2 and long-term durability with no decay after 10 d. The versatile fabrication protocol and smart branch-core design provide a new way to construct other advanced metal sulfides for energy conversion and storage.

Three dimensional metal/N-doped nanoplate carbon catalysts for oxygen reduction, the reason for using a layered nanoreactor

A layered nanoreactor (zinc hydroxide gallate/nitrate nanohybrid) has been designed as a nano-vessel to confine the gallate/nitrate reaction inside zinc hydroxide layers for production of metal/nitrogen-doped carbon catalysts. Metals (Fe²⁺, Co²⁺ and Ni²⁺) doped and bare zinc hydroxide nitrates (ZHN) were synthesized as the α-phase hydroxide hosts. By an incomplete ion-exchange process, nitrate anions between the layers of the hosts were then partially replaced by the gallate anions to produce the layered nanoreactors. Under heat-treatment, the reaction between the remaining un-exchanged nitrate anions and the organic moiety inside the basal spacing of each nanohybrid plate resulted in obtaining highly porous 3D metal/nitrogen-doped carbon nanosheets. These catalysts were then used as extremely efficient electrocatalysts for catalyzing oxygen reduction reaction (ORR). This study is intended to show the way to get maximum electrocatalytic activity of the metal/N-doped carbon catalysts toward the ORR. This exceptionally high ORR performance originates from the increased available surface, the best pore size range and the uniform distribution of the active sites in the produced catalysts, all provided by the use of new idea of the layered nanoreactor.

Template-assisted in situ confinement synthesis of nitrogen and oxygen co-doped 3D porous carbon network for high-performance sodium-ion battery anode

Nitrogen and oxygen co-doped porous carbon network is synthesized by NaCl template-assisted in situ confinement method and used for a high-performance sodium-ion battery anode.

Facile Synthesis of N-Doped Graphene-Like Carbon Nanoflakes as Efficient and Stable Electrocatalysts for the Oxygen Reduction Reaction

A series of N-doped carbon materials (NCs) were synthesized by using biomass citric acid and dicyandiamide as renewable raw materials via a facile one-step pyrolysis method. The characterization of microstructural features shows that the NCs samples are composed of few-layered graphene-like nanoflakes with controlled in situ N doping, which is attributed to the confined pyrolysis of citric acid within the interlayers of the dicyandiamide-derived g-C₃N₄ with high nitrogen contents. Evidently, the pore volumes of the NCs increased with the increasing content of dicyandiamide in the precursor. Among these samples, the NCs nanoflakes prepared with the citric acid/dicyandiamide mass ratio of 1:6, NC-6, show the highest N content of ~6.2 at%, in which pyridinic and graphitic N groups are predominant. Compared to the commercial Pt/C catalyst, the as-prepared NC-6 exhibits a small negative shift of ~66 mV at the half-wave potential, demonstrating excellent electrocatalytic activity in the oxygen reduction reaction. Moreover, NC-6 also shows better long-term stability and resistance to methanol crossover compared to Pt/C. The efficient and stable performance are attributed to the graphene-like microstructure and high content of pyridinic and graphitic doped nitrogen in the sample, which creates more active sites as well as facilitating charge transfer due to the close four-electron reaction pathway. The superior electrocatalytic activity coupled with the facile synthetic method presents a new pathway to cost-effective electrocatalysts for practical fuel cells or metal-air batteries.

Latest advances in supercapacitors: from new electrode materials to novel device designs

Notably, many significant breakthroughs for a new generation of supercapacitors have been reported in recent years, related to theoretical understanding, material synthesis and device designs. Herein, we summarize the state-of-the-art progress toward mechanisms, new materials, and novel device designs for supercapacitors. Firstly, fundamental understanding of the mechanism is mainly focused on the relationship between the structural properties of electrode materials and their electrochemical performances based on some in situ characterization techniques and simulations. Secondly, some emerging electrode materials are discussed, including metal-organic frameworks (MOFs), covalent organic frameworks (COFs), MXenes, metal nitrides, black phosphorus, LaMnO₃, and RbAg₄I₅/graphite. Thirdly, the device innovations for the next generation of supercapacitors are provided successively, mainly emphasizing flow supercapacitors, alternating current (AC) line-filtering supercapacitors, redox electrolyte enhanced supercapacitors, metal ion hybrid supercapacitors, micro-supercapacitors (fiber, plane and three-dimensional) and multifunctional supercapacitors including electrochromic supercapacitors, self-healing supercapacitors, piezoelectric supercapacitors, shape-memory supercapacitors, thermal self-protective supercapacitors, thermal self-charging supercapacitors, and photo self-charging supercapacitors. Finally, the future developments and key technical challenges are highlighted regarding further research in this thriving field.

Synthesis of Uniquely Structured Yolk-Shell Metal Oxide Microspheres Filled with Nitrogen-Doped Graphitic Carbon with Excellent Li-Ion Storage Performance

Novel structured composite microspheres of metal oxide and nitrogen-doped graphitic carbon (NGC) have been developed as efficient anode materials for lithium-ion batteries. A new strategy is first applied to a one-pot preparation of composite (FeO_x -NGC/Y) microspheres via spray pyrolysis. The FeO_x -NGC/Y composite microspheres have a yolk-shell structure based on the iron oxide material. The void space of the yolk-shell microsphere is filled with NGC. Dicyandiamide additive plays a key role in the formation of the FeO_x -NGC/Y composite microspheres by inducing Ostwald ripening to form a yolk-shell structure based on the iron oxide material. The FeO_x -NGC/Y composite microspheres with the mixed crystal structure of rock salt FeO and spinel Fe₃ O₄ phases show highly superior lithium-ion storage performances compared to the dense-structured FeO_x microspheres with and without carbon material. The discharge capacities of the FeO_x -NGC/Y microspheres for the 1st and 1000th cycle at 1 A g^-1 are 1423 and 1071 mAh g^-1 , respectively. The microspheres have a reversible discharge capacity of 598 mAh g^-1 at an extremely high current density of 10 A g^-1 . Furthermore, the strategy described in this study is generally applied to multicomponent metal oxide-carbon composite microspheres with yolk-shell structures based on metal oxide materials.

Electrostatically Assembled Magnetite Nanoparticles/Graphene Foam as a Binder-Free Anode for Lithium Ion Battery

Lithium ion batteries (LIBs) are promising candidates for energy storage, with the development of novel anode materials. We report the fabrication of Fe₃O₄ nanoparticles/graphene foam via electrostatic assembly and directly utilize it as a binder-free anode for LIBs. Owing to the integrated effect of the well-dispersed Fe₃O₄ nanoparticles and the conductive graphene foam network, such composite exhibited remarkable electrochemical performances. It delivered a large reversible specific capacity reaching to ∼1198 mAh g^-1 at a current density of 100 mA g^-1, a good rate capacity, and an excellent cyclic stability over 400 cycles. This work demonstrated a facile methodology to design and construct high-performance anode materials for LIBs.

Chevrel Phase Mo6 T8 (T = S, Se) as Electrodes for Advanced Energy Storage

With the large-scale applications of electric vehicles in recent years, future batteries are required to be higher in power and possess higher energy densities, be more environmental friendly, and have longer cycling life, lower cost, and greater safety than current batteries. Therefore, to develop alternative electrode materials for advanced batteries is an important research direction. Recently, the Chevrel phase Mo₆ T₈ (T = S, Se) has attracted increasing attention as electrode candidate for advanced batteries, including monovalent (e.g., lithium and sodium) and multivalent (e.g., magnesium, zinc and aluminum) ion batteries. Benefiting from its unique open crystal structure, the Chevrel phase Mo₆ T₈ cannot only ensure rapid ion transport, but also retain the structure stability during electrochemical reactions. Although the history of the research on Mo₆ T₈ as electrodes for advanced batteries is short, there has been significant progress on the design and fabrication of Mo₆ T₈ for various advanced batteries as above mentioned. An overview of the recent progress on Mo₆ T₈ electrodes applied in advanced batteries is provided, including synthesis methods and diverse structures for Mo₆ T₈ , and electrochemical mechanism and performance of Mo₆ T₈ . Additionally, a briefly conclusion on the significant progress, obvious drawbacks, emerging challenges and some perspectives on the research of Mo₆ T₈ for advanced batteries in the near future is provided.

Quinone molecules encapsulated in SWCNTs for low-temperature Na ion batteries

We have performed Li and Na ion charge-discharge experiments of 9,10-phenanthrene quinone (PhQ) molecules encapsulated in single-walled carbon nanotubes (SWCNTs) with mean tube diameters of 1.5 and 2.5 nm at room temperature and also at low temperatures. The Na ion reversible capacity of PhQ encapsulated in the larger diameter SWCNTs, measured at a low temperature of 0 °C, remained as high as that measured at room temperature (RT), while the capacity of PhQ in the smaller diameter SWCNTs at 0 °C was about a half of that at RT. The diameter dependence of the capacity should be attributed to the difference in the interactions between the encapsulated PhQ molecules and the host SWCNTs, which was revealed by Raman peak profile analysis. Charge-transfer reaction from metallic tubes to PhQ molecules encapsulated in the smaller diameter SWCNTs was detected by Raman measurements. The electrostatic interaction between charged SWCNTs and PhQ molecules, induced by the charge-transfer reaction, would partly contribute to the stabilization of PhQ molecules in the smaller diameter SWCNTs, while only van der Waals interaction stabilizes PhQ molecules in the larger diameter SWCNTs. The difference in stability was confirmed by thermogravimetric, x-ray photoelectron spectroscopy, and Raman measurements. Charge-discharge curves of PhQ encapsulated in SWCNTs were also discussed based on the stability difference.

A Phase-Separation Route to Synthesize Porous CNTs with Excellent Stability for Na+ Storage

Porous carbon nanotubes (CNTs) are obtained by removing MoO₂ nanoparticles from MoO₂ @C core@shell nanofibers which are synthesized by phase-segregation via a single-needle electrospinning method. The specific surface area of porous CNTs is 502.9 m² g^-1 , and many oxygen-containing functional groups (COH, CO) are present. As anodes for sodium-ion batteries, the porous CNT electrode displays excellent rate performance and cycling stability (110 mA h g^-1 after 1200 cycles at 5 A g^-1 ). Those high properties can be attributed to the porous structure and surface modification to steadily store Na⁺ with high capacity. The work provides a facile and broadly applicable way to fabricate the porous CNTs and their composites for batteries, catalysts, and fuel cells.

Spontaneous Weaving of Graphitic Carbon Networks Synthesized by Pyrolysis of ZIF-67 Crystals

Three-dimensional (3D) networks of graphitic carbon are promising materials for energy storage and conversion devices because of their high electrical conductivity, which is promoted by the good interconnection between the carbon particles. However, it is still difficult to directly synthesize such carbon networks. Herein, we report the novel synthesis of 3D graphitic carbon networks through the pyrolysis of nanosized ZIF-67 crystals. Interestingly, the unusual effect of downsizing the ZIF-67 crystals and the incorporation of catalytic Co nanoparticles was the spontaneous formation of graphitic networks. The obtained graphitic carbon networks show excellent electrochemical performance for the insertion and extraction of potassium ions.