Co9S8 nanoparticle-decorated carbon nanofibers as high-performance supercapacitor electrodes

This work reported Co₉S₈ nanoparticle-decorated carbon nanofibers (CNF) as a supercapacitor electrode. By using a mild ion-exchange method, the cobalt oxide-based precursor nanoparticles were transformed to Co₉S₈ nanoparticles in a microwave hydrothermal process, and these nanoparticles were decorated onto a carbon nanofiber backbone. The composition of the nanofibers can be readily tuned by varying the Co acetate content in the precursor. The porous carbon nanofibers offered a fast electron transfer pathway while the well dispersed Co₉S₈ nanoparticles acted as the redox center for energy storage. As a result, high specific capacitance of 718 F g⁻¹ at 1 A g⁻¹ can be achieved with optimized Co₉S₈ loading. The assembled asymmetric supercapacitor with Co₉S₈/CNF as the cathode showed a high energy density of 23.8 W h kg⁻¹ at a power density of 0.75 kW kg⁻¹ and good cycling stability (16.9% loss over 10 000 cycles).

Through electrospinning and the ion-exchange method, Co₉S₈ nanoparticle-decorated carbon nanofibers (Co₉S₈/CNF) have been fabricated, and exhibit good supercapacitor performance.     

UIO-66-NH2-derived mesoporous carbon used as a high-performance anode for the potassium-ion battery

As potassium is abundant and has an electronic potential similar to lithium''s, potassium-ion batteries (KIBs) are considered as prospective alternatives to lithium-ion batteries (LIBs). However, the much larger radius of the K ion poses challenges for the potassiation and depotassiation processes when the typical graphite-based anode is used, resulting in poor electrochemical performance. Thus, there is an urgent need to develop novel anode materials that are suitable for K ions. Herein, we develop a porous carbon material with high surface area derived from UIO-66-NH₂ metal–organic frameworks as an anode material instead of a graphite-based anode. The material is prepared using a double-solvent diffusion-pyrolysis method, which increased mesopore volume and average pore size, and to a certain extent, slightly improved the nitrogen content of the production. The material exhibits a high capacity as well as excellent rate performance and cycling stability. A potassium battery with our porous carbon as the anode delivers a high reversible capacity of 346 mA h g⁻¹ at 100 mA g⁻¹ (compared to 279 mA h g⁻¹ with a graphite-based anode), and 214 mA h g⁻¹ at a discharge rate of up to 2 A g⁻¹. After 800 cycles, the capacity is still 187 mA h g⁻¹ at 0.1 A g⁻¹. Qualitative and quantitative kinetics analyses demonstrated that the battery''s high K storage performance was principally dominated by a surface-driven capacitive mechanism, and the potassiation and depotassiation processes may have occurred on the surface of the porous carbon instead of in the interlayer space, as is the case with a graphite anode. This work may provide a basis for developing other carbonaceous materials to use in KIBs.

The N-doped mesoporous carbon material prepared by a double-solvent diffusion pyrolysis method with UIO-66-NH₂ as a precursor can deliver a high reversible capacity of 346 mA h g⁻¹ at 100 mA g⁻¹ when used as an anode for non-aqueous KIBs.     

A graphite-modified natural stibnite mineral as a high-performance anode material for sodium-ion storage

Recently, Sb₂S₃ has drawn extensive interest in the energy storage domain due to its high theoretical capacity of 946 mA h g⁻¹. However, the inherent disadvantages of serious volume expansion and poor conductivity restrict the development of Sb₂S₃ for its application in SIBs. In addition, chemical synthesis is a main method to prepare Sb₂S₃, which is commonly accompanied by environmental pollution and excessive energy consumption. Herein, the natural stibnite mineral was directly applied in SIBs after modification with graphite via an effective and facile approach. The novel composites exhibited excellent electrochemical properties with higher reversible capacity, better rate capability and more outstanding cycling stability than the bare natural stibnite mineral. Briefly, this study is anticipated to provide a reference for the development of natural minerals as first-hand materials in energy storage and a new approach to improve natural stibnite mineral composites for their application as anodes in SIBs.

The natural stibnite mineral modified with graphite provides a reference for the development of natural mineral as first-hand materials in energy storage and a new approach to improve natural stibnite mineral composites as anode in SIBs.     

Hard carbon spheres prepared by a modified Stöber method as anode material for high-performance potassium-ion batteries

The Stöber method is a highly efficient synthesis strategy for homogeneous monodisperse polymer colloidal spheres and carbon spheres. This work delivers an extended Stöber method and investigates the synthesis process. By calcining the precursor under appropriate conditions, solid secondary particles of amorphous carbon (SSAC) and hollow secondary particles of graphitized carbon (HSGC) can be directly synthesized. The two materials have a nano-primary particle structure and a closely-packed sub-micron secondary particle structure, which can be used in energy storage. We find that SSAC and HSGC have high potassium-ion storage capacity with reversible capacities of 274 mA h g⁻¹ and 283 mA h g⁻¹ at 20 mA g⁻¹ respectively. Significantly, SSAC has better rate performance with a specific capacity of 107 mA h g⁻¹ at 1 A g⁻¹.

A modified Stöber method synthesizes resorcinol-formaldehyde resin-based secondary particle hard carbon spheres as anode material for high-performance potassium-ion batteries.     

S-doped porous carbon anode with superior capacity for high-performance sodium storage

Heterogeneous carbon-based materials with high porosity are attracting increased attention for energy storage due to their enhanced capacity and rate performance. Herein, we report a sulfur-doped porous carbon material, which is achieved by spray-drying and subsequent sulfuration. The porous structure can provide vast diffusive tunnels for the fast access of electrolytes and sodium ions. Also, the S–C bond increases the electrical conductivity of the carbon frameworks and offers excessive reaction sites for sodium-ion storage. The elaborated carbon architecture enables a high capacity of 370 mA h g⁻¹ at 0.5 A g⁻¹ and provides an excellent rate performance for long-term cycling (197 mA h g⁻¹ at 2.0 A g⁻¹ for 650 cycles). Considering the scalable and facile spray-pyrolysis preparation route, this material is expected to serve as a low-cost and environmentally friendly anode for practical sodium-ion batteries.

A novel S-doped porous carbon material with superior sodium storage performance was obtained through spray-drying and subsequent sulfur doping treatment.     

Microflower-like Co9S8@MoS2 heterostructure as an efficient bifunctional catalyst for overall water splitting

The development of a distinguished and high-performance catalyst for H₂ and O₂ generation is a rational strategy for producing hydrogen fuel via electrochemical water splitting. Herein, a flower-like Co₉S₈@MoS₂ heterostructure with effective bifunctional activity was achieved using a one-pot approach via the hydrothermal treatment of metal-coordinated species followed by pyrolysis under an N₂ atmosphere. The heterostructures exhibited a 3D interconnected network with a large electrochemical active surface area and a junctional complex with hydrogen evolution reaction (HER) catalytic activity of MoS₂ and oxygen evolution reaction (OER) catalytic activity of Co₉S₈, exhibiting low overpotentials of 295 and 103 mV for OER and HER at 10 mA cm⁻² current density, respectively. Additionally, the catalyst-assembled electrolyser provided favourable catalytic activity and strong durability for overall water splitting in 1 M KOH electrolyte. The results of the study highlight the importance of structural engineering for the design and preparation of cost-effective and efficient bifunctional electrocatalysts.

A flower-like Co₉S₈@MoS₂ heterostructure was prepared as efficient bifunctional electrocatalyst for overall water splitting by a sample one-pot approach.     

First-principles computational investigation of nitrogen-doped carbon nanotubes as anode materials for lithium-ion and potassium-ion batteries

Significant research efforts, mostly experimental, have been devoted to finding high-performance anode materials for lithium-ion and potassium-ion batteries; both graphitic carbon-based and carbon nanotube-based materials have been generating huge interest. Here, first-principles calculations are performed to investigate the possible effects of doping defects and the varying tube diameter of carbon nanotubes (CNTs) on their potential for battery applications. Both adsorption and migration of Li and K are studied for a range of pristine and nitrogen-doped CNTs, which are further compared with 2D graphene-based counterparts. We use detailed electronic structure analyses to reveal that different doping defects are advantageous for carbon nanotube-based and graphene-based models, as well as that curved CNT walls help facilitate the penetration of potassium through the doping defect while showing a negative effect on that of lithium.

First-principles simulations reveal atomistic mechanisms for adsorption and migration of lithium and potassium on nitrogen-doped carbon nanotubes.     

A Co-MOF-derived Co9S8@NS-C electrocatalyst for efficient hydrogen evolution reaction

The exploitation of efficient hydrogen evolution reaction (HER) electrocatalysts has become increasingly urgent and imperative; however, it is also challenging for high-performance sustainable clean energy applications. Herein, novel Co₉S₈ nanoparticles embedded in a porous N,S-dual doped carbon composite (abbr. Co₉S₈@NS-C-900) were fabricated by the pyrolysis of a single crystal Co-MOF assisted with thiourea. Due to the synergistic benefit of combining Co₉S₈ nanoparticles with N,S-dual doped carbon, the composite showed efficient HER electrocatalytic activities and long-term durability in an alkaline solution. It shows a small overpotential of −86.4 mV at a current density of 10.0 mA cm⁻², a small Tafel slope of 81.1 mV dec⁻¹, and a large exchange current density (J₀) of 0.40 mA cm⁻², which are comparable to those of Pt/C. More importantly, due to the protection of Co₉S₈ nanoparticles by the N,S-dual doped carbon shell, the Co₉S₈@NS-C-900 catalyst displays excellent long-term durability. There is almost no decay in HER activities after 1000 potential cycles or it retains 99.5% of the initial current after 48 h.

A porous Co₉S₈@NS-C-900 composite was fabricated by the pyrolysis of crystal Co-MOF involving thiourea. The composite exhibits efficient electrocatalytic activities and long-term durabilities towards HER in alkaline electrolytes.     

Correction: Microflower-like Co9S8@MoS2 heterostructure as an efficient bifunctional catalyst for overall water splitting

Correction for ‘Microflower-like Co₉S₈@MoS₂ heterostructure as an efficient bifunctional catalyst for overall water splitting’ by Chaohai Pang et al., RSC Adv., 2022, 12, 22931–22938, https://doi.org/10.1039/D2RA04086G.

The authors regret that an incorrect grant number was shown in the acknowledgements section of the published article. The corrected section should read:This work was financially supported by the Central Public-interest Scientific Institution Basal Research Fund (No.1630082022008). China Agriculture Research System of MOF and MARA (CARS-31). Key Laboratory of Tropical Fruits and Vegetables Quality and Safety for State Market Regulation (Grant no. ZX-2022002). Additional support was provided by the Foundation from Chemistry and Chemical Engineering Guangdong Laboratory (Grant No. 2111016) and the Basic and Applied Basic Research Foundation of Guangdong Province (Grant no. 2021A1515110111).The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.     

Li2ZnTi3O8/graphene nanocomposite as a high-performance anode material for lithium-ion batteries

An Li₂ZnTi₃O₈/graphene (LZTO/G) anode is successfully synthesized by a two-step reaction. The results show that LZTO particles can be well dispersed into the graphene conductive network. The conductive structure greatly improves the electrochemical performance of LZTO/G. When cycled for 400 cycles, 76.4% of the capacity for the 2nd cycle is maintained at 1 A g⁻¹. Also, 174.8 and 156.5 mA h g⁻¹ are still delivered at the 100th cycle for 5 and 6 A g⁻¹, respectively. The excellent cyclic performance and the large specific capacities at high current densities are due to the good conductive network of the LZTO active particles, large pore volume, small particle size, low charge-transfer resistance and high lithium diffusion coefficient.

The structure of G modifying LZTO is beneficial for the diffusion of Li⁺ ions and transportation of electrons.     

Zinc vacancy modulated quaternary metallic oxynitride GeZn1.7ON1.8: as a high-performance anode for lithium-ion storage

The development of alternative anode materials to achieve high lithium-ion storage performance is crucial for the next-generation lithium-ion batteries (LIBs). In this study, a new anode material, Zn-defected GeZn_1.7ON_1.8 (GeZn_1.7−xON_1.8), was rationally designed and successfully synthesized by a simple ammoniation and acid etching method. The introduced zinc vacancy can increase the capacity by more than 100%, originating from the additional space for the lithium-ion insertion. This GeZn_1.7−xON_1.8 particle anode delivers a high capacity (868 mA h g⁻¹ at 0.1 A g⁻¹ after 200 cycles) and ultralong cyclic stability (2000 cycles at 1.0 A g⁻¹ with a maintained capacity of 458.6 mA h g⁻¹). Electrochemical kinetic analysis corroborates the enhanced pseudocapacitive contribution and lithium-ion reaction kinetics in the GeZn_1.7−xON_1.8 particle anode. Furthermore, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses at different electrochemical reaction states confirm the reversible intercalation lithium-ion storage mechanism of this GeZn_1.7−xON_1.8 particle anode. This study offers a new vision toward designing high-performance quaternary metallic oxynitride-based materials for large-scale energy storage applications.

Zn-defected GeZn_1.7ON_1.8 (GeZn_1.7−xON_1.8) was successfully synthesized by a simple ammoniation and acid etching method. This well-designed Zn cation-deficient GeZn_1.7−xON_1.8 anode shows enhanced lithium-ion storage performance.     

Three-dimensional TiNb2O7 anchored on carbon nanofiber core–shell arrays as an anode for high-rate lithium ion storage

The control of structure and morphology in an electrode design for the development of large-power lithium ion batteries is crucial to create efficient transport pathways for ions and electrons. Herein, we report a powerful combinational strategy to build omnibearing conductive networks composed of titanium niobium oxide nanorods and carbon nanofibers (TNO/CNFs) via an electrostatic spinning method and a hydrothermal method into free-standing arrays with a three-dimensional heterostructure core/shell structure. TNO/CNF electrode exhibits significantly superior electrochemical performance and high-rate capability (241 mA h g⁻¹ at 10C, and 208 mA h g⁻¹ at 20C). The capacity of the TNO/CNF electrode is 257 mA h g⁻¹ after 2000 cycles at 20C, which is much higher than that of the TNO electrode. In particular, the TNO/CNF electrode delivers a reversible capacity of 153.6 mA h g⁻¹ with a capacity retention of 95% after 5000 cycles at ultrahigh current density. Superior electrochemical performances of the TNO/CNF electrode are attributed to the unique composite structure.

The control of structure and morphology in an electrode design for the development of large-power lithium ion batteries is crucial to create efficient transport pathways for ions and electrons.     

Ultrafine MoO2 nanoparticles encapsulated in a hierarchically porous carbon nanofiber film as a high-performance binder-free anode in lithium ion batteries

Flexible free-standing hierarchically porous carbon nanofibers embedded with ultrafine (∼3.5 nm) MoO₂ nanoparticles (denoted as MoO₂@HPCNFs) have been synthesized by electrospinning and subsequent heat treatment. When evaluated as a binder-free anode in Li-ion batteries, the as-obtained MoO₂@HPCNFs film exhibits excellent capacity retention with high reversible capacity (≥1055 mA h g⁻¹ at 100 mA g⁻¹) and good rate capability (425 mA h g⁻¹ at 2000 mA g⁻¹), which is much superior to most of the previously reported MoO₂-based materials. The synergistic effect of uniformly dispersed ultrasmall MoO₂ nanoparticles and a three-dimensionally hierarchical porous conductive network constructed by HPCNFs effectively improve the utilization rate of active materials, enhance the transport of both electrons and Li⁺ ions, facilitate the electrolyte penetration, and promote the Li⁺ storage kinetics and stability, thus leading to a greatly enhanced electrochemical performance.

A novel binder-free LIB anode made of ultrafine MoO₂ nanoparticles encapsulated in hierarchically porous carbon nanofibers exhibits high Li-storage performance.     

Designed formation of Co3O4@NiCo2O4 sheets-in-cage nanostructure as high-performance anode material for lithium-ion batteries

Structural and compositional control of functional nanoparticles is considered to be an efficient way to obtain enhanced chemical and physical properties. A unique Co₃O₄@NiCo₂O₄ sheets-in-cage nanostructure is fabricated via a facile conversion reaction, involving subsequent hydrolysis and annealing treatment. Such hollow nanoparticles provide an excellent property for Li storage.

Unique Co₃O₄@NiCo₂O₄ sheets-in-cage hybrid nanoparticles are successfully fabricated through a template-assisted method. When evaluated as an anode material, they exhibit highly enhanced electro-chemical properties for lithium storage.

Recently, the development of low-cost and environment-friendly renewable energy conversion and storage systems has become a research focus to address the increasingly serious energy crisis and environment issues.^1–3 Lithium-ion batteries (LIBs), as one of the most important energy storage technologies, have received tremendous attention in the past decades.^4–8 Among the various anode materials, Co-based transition metal oxides have been widely investigated owing to their higher theoretical capacity compared with the traditional anode electrode, graphite (372 mA h g⁻¹).^9–13 However, such kind of anodes usually suffer from the large volume changes during the long-term charge/discharge process, as well as the low conductivity, which are the main reasons for the fast fading in capacity and poor rate performance.One efficient way to solve the problem is the construction of complex hollow nanostructures instead of traditional solid ones.^14–17 The complex hollow nanostructures have some unique advantages, such as the larger specific surface area, well-defined inner hollow space and increased surface permeability. These factors are favourable for the fast mass/charge transfer, resulting in highly increased property for lithium storage. To date, many kinds of synthetic strategies have been successfully developed and reported. As examples, Wang and co-workers reported a facile synthesis of multi-shelled Co₃O₄ nanospheres with controllable shells.¹⁸ Carbon nanospheres are pre-synthesized and used as the hard templates to adsorb the Co²⁺ ions, followed by an annealing treatment in air for the final conversion of Co₃O₄ crystals. Lou''s group synthesized a series of Co-based complex hollow nanostructures.^19–22 Co₃O₄@NiCo₂O₄,¹⁹ Co₃O₄@Co–Fe mixed oxide²⁰ and Co₃O₄@Co₃V₂O₈ triple-shelled nanocages²¹ are successfully fabricated. Otherwise, Co₃O₄ bubbles/N doped carbon,²³ Co₃O₄ hollow nanospheres with ultra-thin nanosheets as building blocks,²⁴ and Co₃O₄/N doped carbon nanocubes²⁵ are also prepared. Benefiting from the unique structural features, these hollow materials always show highly enhanced electro-chemical properties. In addition, using heteroatoms to couple with Co metal to form the mixed metal compound is considered as another effective way to increase the LIB performance. Such mixed metal compounds are favourable for controlling the local chemical environment and modulating the synergistic effect between etch component. More importantly, the coupling of different kinds of metal ions together can bring richer redox reactions and higher electronic conductivity, resulting in significant enhancement of the LIB performance.^26–31 NiCo₂O₄ hollow spheres,^27,28 hollow polyhedrons,²⁹ nanoarrays,³⁰ and ZnCo₂O₄/carbon cloth hybrids³¹ are fabricated and used as high-performance anodes for LIBs. Therefore, a rational design and synthesis of Co-based functional nanomaterials with complex inner hollow nanostructure and multi-component-coupled outer surface is expected to yield the increased LIB performance.In the past two decades, metal–organic frameworks (MOFs) have been widely used as sacrificial templates for the synthesis of functional materials.^{19–21,32–39} The MOFs have some distinctive structural and compositional advantages, such as high porosity and easily controllable composition. Furthermore, the chemical and thermal instability makes the conversion reaction occur easily, that MOFs can transform to functional nanoparticles via a facile ion-exchange reaction or annealing treatment in a specific atmosphere. Among the variety of MOFs, Co-based zeolitic imidazolate framework-67 (ZIF-67) has received continues attention and been widely studied. As successful examples, 2-methylimidazole ligand in ZIF-67 can be easily replaced by S²⁻, Se²⁻ and MoO₄²⁻ to form the corresponding CoS,³⁷ CoSe,³⁸ and CoMoO₄ hollow cages.³⁹ Such MOF-assisted synthetic strategy provides more possibilities to obtain functional materials with improved physical and chemical properties.Following the above considering, a new sheets-in-cage nanostructure composed by Co₃O₄ nanosheets inside and NiCo₂O₄ layer as outer shell has been designed and synthesized. The whole synthesis involves: (i) conversion reaction to produce ZIF-67@NiCo-LDH yolk@shell nanoparticles; (ii) hydrolysis reaction between ZIF-67@NiCo-LDH and hexamethylenetetramine (HMT) to form thin Co(OH)₂ nanosheets in the NiCo-LDH nanocage; (iii) annealing treatment in air atmosphere at 300 °C for 2 hours for the fabrication of final Co₃O₄@NiCo₂O₄ sheets-in-cage hollow nanoparticles. In our design, three important structural advantages should be pointed out. First, strongly coupled NiCo₂O₄ as outer shell is expected to bring higher electro-conductivity. Second, the inner hollow space could accommodate the large volume changes during the lithiation/delithiation process. Last, the encapsulated Co₃O₄ nanosheets may provide more active sits for lithium storage. As expected, in the following LIB application, the as-obtained Co₃O₄@NiCo₂O₄ sheets-in-cage nanoparticles exhibit enhanced LIB performance.The ZIF-67 crystals are synthesized according to the previous report.¹⁹ Scanning electron microscopy (SEM) transmission electron microscopy (TEM) and powder X-ray diffraction (XRD) are used as characterization methods. As shown in Fig. S1,^† the product is uniform polyhedron with a particle size of ≈800 nm in average. The corresponding XRD pattern in Fig. S2^† (black line) matches well with the previous reports,^19,37,38 indicating the product is in the pure phase of ZIF-67. Next, the above obtained ZIF-67 crystals are further reacted with Ni(NO₃)₂ in absolute ethanol. 40 min later, the products are collected and washed with ethanol for three times (details are shown in the ESI^†).With the data displayed in Fig. 1a–c, the as-obtained particles are uniform and monodisperse. Compared with the original ZIF-67 templates, they keep the dodecahedral morphology with the similar average particle size (≈800 nm). However, the outer surface becomes much rougher. Ultra-thin nanosheet as the building block can be clearly observed. A solid core can be found in each particle. And the clear gap is formed between the shell and solid core, indicating the formation of yolk@shell nanostructure. The corresponding XRD pattern is shown in Fig. S2^† (red line). The specific diffraction peaks can be assigned to ZIF-67 crystal. By referring to the report,⁴⁰ it is considered Co²⁺ is slowly dissolved from ZIF-67 in such synthesis, then oxidized to Co³⁺ by NO₃⁻. Further co-precipitation with Ni²⁺ causes the formation of NiCo-LDH. Along with the hydrolysis reaction, H⁺ is generated, and ZIF-67 is etched continuously. As a result, ZIF-67@NiCo-LDH yolk@shell nanostructure is obtained. Further prolonging the reaction time to three hours can cause the complete dissolution of the inner ZIF-67 core. As revealed by the TEM images in Fig. S3,^† single-shelled NiCo-LDH nanocages are successfully produced.Open in a separate windowFig. 1TEM images of ZIF-67@NiCo-LDH yolk@shell nanoparticles (a to c) and Co(OH)₂@NiCo-LDH sheets-in-cage nanoparticles (d to f).In next step, HMT is chosen as the weak alkali source. After heated at 90 °C for two hours, the color of the solution becomes from purple to yellow, indicating the complete conversion of inner ZIF-67 core. TEM images are first used to investigate the structure information. As shown in Fig. 1d–f, there is no obverse change in appearance. The overall structure is well maintained. The particles have the similar particle size, the outer surface is still rough and the thin nanosheets are clear to be observed. As expected, the inner solid cores are disappeared. Unexpected, large amount of thicker Co(OH)₂ sheets are formed and criss-cross in the nanocage. The formation mechanism could be understood as follow: OH⁻ ions are first generated from HMT at high temperature, and then reacted with ZIF-67 to form the Co(OH)₂ nanosheets. The hollow space inside is large enough and the outer NiCo-LDH layer is stable to limit the expanded growth of Co(OH)₂ nanosheets, thus, the unique sheets-in-cage hybrid nanostructure is successfully synthesized. The XRD pattern of the as-obtained product is investigated and shown in Fig. S4.^† The specific diffraction peaks of the solid ZIF-67 crystals disappeared completely, no clear diffraction peaks could be found, indicating the poor crystallization degree of the product.Some control experiments are carried out to get a deeper understand about the formation mechanism. First, we tried to study the solvent-effect towards the structure evolution. Pure ethanol is used to replace the ethanol/water mixture. Two hours later, the colour of the solution is still purple, indicating the remain of ZIF-67. Fig. S5^† displays the typical TEM images, that the change of the inner solid core is not obvious. Otherwise, there is no Co(OH)₂ nanosheets formed during the process. It reveals that water is necessary, which can participate in the decomposition of HMT to release OH⁻ ions. Then, we increase the volume ratio of H₂O/ethanol to 2/1. With the TEM images shown in Fig. S6,^† the sheets-in-cage hybrid nanostructure is also obtained. The similar phenomenon is also appeared when doubling the feeding amount of HMT (Fig. S7^†). In such two conditions, large amount of OH⁻ ions are generated, and the inner ZIF-67 cores can be dissolved completely. A control experiment is also done by using pure water as the solvent. In such condition, no precipitation is obtained. It is considered that OH⁻ is generated fast, which causes the seriously damage of the three-dimensional cages. Thus, ultra-thin Co/Ni hydroxide nanosheets are well dispersed in water and hard to be collected by centrifugation. Next, we tried to use ZIF-67 crystals to instead of ZIF-67@NiCo-LDH yolk@shell nanoparticles to react with HMT directly. Four hours later, the original purple solution becomes to pink. The TEM images in Fig. S8^† reveal that the as-obtained hollow particles show significant shrinkage with decreased size compared with the original ZIF-67 templates. No criss-crossed nanosheets formed inside the cages.Such phenomenon indicates the outer NiCo-LDH layer is important for the formation of such sheets-in-cage nanostructure. The difference of crystal structure between NiCo-LDH and Co(OH)₂ prevents the expanded growth of Co(OH)₂ nanosheets. Otherwise, we also study the alkali effect. NH₃H₂O is used to instead of HMT. The TEM images in Fig. S9^† reveal that the inner hollow space cannot be maintained in such condition. Small black particles are formed and loaded on the surface of the nanosheets. However, when urea is used, similar sheets-in-cage hybrids are successful synthesized. Urea is much similar with HMT, they can provide OH⁻ ions slowly, which is considered as the necessary condition for the formation of inner nanosheets.Finally, the pre-synthesized sample is annealed in air at 300 °C for the formation of crystallized product. SEM, TEM, XRD and XPS have been used to characterize the structural and composition information. The typical SEM image is displayed in Fig. 2a and b. The particles can withstand the annealing treatment. Compared with the un-heated precursor, the polyhedron morphology has been maintained well with rough surface. The inner nanostructure is further investigated by TEM images. With the data shown in Fig. 2d–f, the hollow cage is filled with thin Co₃O₄ nanosheets. XRD pattern of the annealed products are taken and shown in Fig. 2c. Three diffraction peaks appear at 2θ = 31.2°, 36.8°, 44.8°, 59.4° and 65.3° which can be indexed to the (220), (311), (400), (511) and (400) planes of Co₃O₄. In addition, the composition of the product is further determined by EDX analysis. As shown in Fig. S10,^† Co and Ni signals are clear observed. X-ray photoelectron spectroscopy (XPS) is further used to determine the chemical state of Co, Ni and O in Co₃O₄/NiCo₂O₄ sheets-in-cage sample. With the data shown in Fig. 3, there are two peaks appear at 779 eV and 795 eV, corresponding to the Co 2p_3/2 and 2p_1/2 spin orbit peaks, respectively. As shown in Fig. 3c, two strong peaks are observed at 854 and 872 eV, which could be assigned to the Ni 2p_3/2 and 2p_1/2 spin orbit, respectively. These results reveal that the presence of Co²⁺, Co³⁺ and Ni²⁺ in the final product. Otherwise, the contents of Co and Ni are also calculated based on the XPS analysis, that the molar ratio of Co/Ni is ≈23/6. For better comparison, NiCo-LDH cages and ZIF-67 are also annealed in air under the same condition. With the typical TEM images shown in Fig. S11 and S12,^† the single-shelled NiCo₂O₄ and Co₃O₄ nanocages are obtained.Open in a separate windowFig. 2SEM (a and b), TEM images (d to f) and XRD pattern (c) of Co₃O₄@NiCo₂O₄ sheets-in-cage hybrid nanoparticle.Open in a separate windowFig. 3XPS spectra of Co₃O₄@NiCo₂O₄ sheets-in-cage hybrid nanoparticles.Next, we further test the Li storage performance of the as-obtained Co₃O₄@NiCo₂O₄ sheets-in-cage nanoparticles. Fig. 4a displays the first four cyclic voltammogram (CV) curves of the Co₃O₄@NiCo₂O₄ electrode at a scan rate of 0.1 mV s⁻¹ in the potential window of 3 V to 0.01 V. There is a big difference between the first cycle and the others. It is clear two reduction peaks appeared at about 1.3 V and 0.8 V in the first discharge process, corresponding to the destruction and/or amorphization of their crystal structure and the reduction reaction between Co₃O₄/NiCo₂O₄ and Li, respectively.^41,42 During this process, irreversible solid electrolyte interface is also generated. For the first charge process, two wide peaks at 1.5 and 2.2 V are clearly observed, which could be attributed to the oxidation reaction between Co/Ni metal and Li_xO. Starting from the second cycle, there is an obverse shift of the main reduction peak from 0.8 to 1.0 V, which could be assign to the irreversible processes in first cycle. Importantly, the following CV curves overlap well, indicating the excellent reversibility and stability.Open in a separate windowFig. 4Cyclic voltammogram measurements at a scan rate of 0.1 mV s⁻¹ (a), charge–discharge voltage profiles in the first and second cycles at a current density of 100 mA g⁻¹ (b) and rate performance at various current densities (d) of the Co₃O₄@NiCo₂O₄ sheets-in-cage electrodes; cycling performances (c) of Co₃O₄@NiCo₂O₄, NiCo₂O₄ and Co₃O₄ nanocages at a current density of 100 mA g⁻¹ for 100 successfully cycles (discharged capacity). Fig. 4b is the charge–discharge voltage profiles of the Co₃O₄@NiCo₂O₄ sheets-in-cage nanocages at a current density of 100 mA g⁻¹ for 1st and 2nd cycle. In the first cycle, the discharge voltage plateaus related the reduction reaction of Co₃O₄/NiCo₂O₄ and the charge voltage about the corresponding oxidation reaction appears at 1.2 and 2.1 V, respectively. Starting from the second cycle, the discharge voltage plateau is shifted, indicating the changes of the structure during the charging and discharging process. These results match well with the CV measurements. Fig. 4c shows the cycling performance of Co₃O₄@NiCo₂O₄ sheets-in-cage nanocages in the current density of 100 mA g⁻¹. In the first discharge process, an ultra-high capacity of 1262 mA h g⁻¹ with a coulombic efficiency of 62% is achieved. After then, the second cycle delivers a capacity of 959 mA h g⁻¹. The capacity is slightly decreased in the following ten cycles, and then increased to 1083 mA h g⁻¹ after 100 cycles. The increase of the capacities could be attributed to the activation effect. The electrode material can become more and more accessible to host lithium ions during the charge/discharge process.^43–45 Furthermore, each cycle exhibits a high coulombic efficiency of around 99% excepted the first cycle. For comparison, single-shelled NiCo₂O₄ and Co₃O₄ nanocages are also used as anode materials for LIBs. As shown in Fig. 4c blue line, NiCo₂O₄ nanocage delivers a specific capacity of 882 mA h g⁻¹ after 100 cycles at a current density of 100 mA g⁻¹. And the capacity of Co₃O₄ nanocage fads fast, a capacity of only 626 mA h g⁻¹ is obtained after 100 cycles (Fig. 4c black line). In addition, the long-term cycling performance of Co₃O₄@NiCo₂O₄ sheets-in-cage nanocage at higher current density of 1 A g⁻¹ is also tested. With the data displayed in Fig. S13,^† a high capacity of 658 mA h g⁻¹ is remained after 200 successful cycles. We also collect the Co₃O₄@NiCo₂O₄ sheets-in-cage samples after 30 cycles at the current density of 100 mA g⁻¹. The corresponding TEM images are shown in Fig. S14.^† Although the shrink of the nanostructure is observed, the overall sheets-in-cage hollow nanostructure is maintained well, indicating the high structural stability.The rate performance is another important factor for anode materials. Fig. 4d shows the rate performance of the as-obtained Co₃O₄@NiCo₂O₄ sheets-in-cage nanocage by gradually increasing the current density from 0.1 to 5 A g⁻¹ and then returning it to 0.1 A g⁻¹. It delivers specific capacities of 942, 872, 786, 734, 668, and 578 mA h g⁻¹ at the current densities of 0.1, 0.2, 0.5, 1, 2 and 5 A g⁻¹, respectively. Impressively, when the current density returned to 100 mA g⁻¹, the capacity is fast increased to 890 mA h g⁻¹, indicating the excellent rate performance and structural stability. For comparison, the average discharge capacities of the as-obtained NiCo₂O₄ and Co₃O₄ nanocages are measured to be 926, 845, 760, 705, 642, 550, and 735, 587, 542, 503, 475, 311 mA h g⁻¹ at a current density of 0.1, 0.2, 0.5, 1, 2 and 5 A g⁻¹ respectively, which is much lower than the Co₃O₄@NiCo₂O₄ sheets-in-cage material (with the data shown in Fig. S15^†).Electrochemical impedance spectroscopy (EIS) is adopted to further investigate the electrochemical kinetics difference among the as-obtained three samples. With the data shown in Fig. S16,^† the Nyquist plots are consisted of a semicircle at the high to medium frequencies followed by a sloped line at the low frequencies. It is obvious that Co₃O₄@NiCo₂O₄ sheets-in-cage electrode has the lowest charge transfer resistance, which could be a big benefit to the LIB performance.Such enhanced LIB property of the Co₃O₄@NiCo₂O₄ sheets-in-cage nanocages could be attributed to the following features. First, the inner thin Co₃O₄ nanosheets have larger surface area, which can provide massive active sites for lithium storage. Second, the formation of hollow space in NiCo₂O₄ nanocages is considered as the efficient way to alleviate the volume change and buffer the induced strain during quick charge/discharge process. Thirdly, the outer surface is strongly coupled NiCo₂O₄ mixed metal oxide, which is expected to bring higher electro-conductivity compared with the Co₃O₄ component.     

ZrO2 coated Li1.9K0.1ZnTi3O8 as an anode material for high-performance lithium-ion batteries

The Li_1.9K_0.1ZnTi₃O₈@ZrO₂ (1 wt%, 3 wt%, and 5 wt%) anode material was synthesized by doping Li₂ZnTi₃O₈ with potassium and coating ZrO₂, where the ZrO₂ coating layer was prepared by citric acid and zirconium acetate, and the potassium source was KCl. When the added ZrO₂ amount is 3%, the material has the most uniform size, reduced polarization, and reduced charge transfer resistance, and the specific capacity of LKZTO@ZrO₂ (3 w%) was 361.5 mA h g⁻¹ at 200 mA g⁻¹ at the 100th cycle, which is higher than that of LKZTO, of 311.3 mA h g⁻¹. The specific capacities of LKZTO@ZrO₂ (3 w%) at 50, 100, 200, 500, and 1000 mA g⁻¹ after 10 cycles were 424.9, 410.7, 394.1, 337.6 and 270.6 mA h g⁻¹, indicating that LKZTO@ZrO₂ (3 w%) has excellent electrochemical performance.

The Li_1.9K_0.1ZnTi₃O₈@ZrO₂ (1 wt%, 3 wt%, and 5 wt%) anode material was successfully synthesized, where the ZrO₂ coating layer was prepared by citric acid and zirconium acetate, and the potassium source was KCl.     

Three-dimensional thiophene-diketopyrrolopyrrole-based molecules/graphene aerogel as high-performance anode material for lithium-ion batteries

Herein, 3,6-di(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (TDPP) and di-tert-butyl 2,2′-(1,4-dioxo-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-2,5(1H,4H)-diyl)diacetate (TDPPA) were synthesized, which were then loaded in graphene aerogels. The as-prepared thiophene-diketopyrrolopyrrole-based molecules/reduced graphene oxide composites for lithium-ion battery (LIB) anode composites consist of DPPs nanorods on a graphene network. In relation to the DPPs part, embedding DPPs nanorods into graphene aerogels can effectively reduce the dissolution of DPPs in the electrolyte. It can serve to prevent electrode rupture and improve electron transport and lithium-ion diffusion rate, by partially connecting DPPs nanorods through graphene. The composite not only has a high reversible capacity, but also shows excellent cycling stability and performance, due to the densely distributed graphene nanosheets forming a three-dimensional conductive network. The TDPP₆₀ electrode exhibits high reversible capacity and excellent performance, showing an initial discharge capacity of 835 mA h g⁻¹ at a current density of 100 mA g⁻¹. Even at a current density of 1000 mA g⁻¹, after 500 cycles, it still demonstrates a discharge capacity of 303 mA h g⁻¹ with a capacity retention of 80.7%.

Herein, TDPP and TDPPA were synthesized and then loaded on graphene by hydrothermal method to obtain TDPP/RGO and TDPPA/RGO aerogel, which were applied as anode for LiBs.     

Structure and conductivity enhanced treble-shelled porous silicon as an anode for high-performance lithium-ion batteries

Silicon is regarded as the next generation anode material for lithium-ion batteries because of its high specific capacity, low intercalation potential and abundant reserves. However, huge volume changes during the lithiation and delithiation processes and low electrical conductivity obstruct the practical applications of silicon anodes. In this study, a treble-shelled porous silicon (TS-P-Si) structure was synthesized via a three-step approach. The TS-P-Si anode delivered a capacity of 858.94 mA h g⁻¹ and a capacity retention of 87.8% (753.99 mA h g⁻¹) after being subjected to 400 cycles at a current density of 400 mA g⁻¹. The good cycling performance was due to the unique structure of the inner silicon oxide layer, middle silver nano-particle layer and outer carbon layer, leading to a good conductivity and a decreased volume change of this silicon-based anode.

In this paper, a treble-shelled porous silicon structure is synthesized through three-step approach to enhance the structural stability and conductivity.     

A reclaimed piezoelectric catalyst of MoS2@TNr composites as high-performance anode materials for supercapacitors

A piezoelectric catalyst of the MoS₂@TNr composite (MoS₂ nanosheets composited with TiO₂ nanorods) was synthesized by a two-step hydrothermal method, and can be recycled and reused as an advanced anode material for supercapacitors. In the dark, the MoS₂@TNr composite exhibited ultra-fast piezoelectric catalytic performance and good cycle stability on dye degradation; within 10 min, nearly all rhodamine B (50 mL, 20 ppm) was removed from the solution with the assistance of magnetic stirring. After the 5 cycle degradation reaction, the catalyst was reclaimed and applied to electrochemical testing, which showed better supercapacitor capacitance properties than the fresh catalyst due to the introduction of oxygen vacancies generated from the piezoelectric degradation process. The reclaimed catalyst demonstrated an excellent specific capacitance of 249 F g⁻¹ at 1 A g⁻¹, and 92% capacitance retention after 10 000 cycles. Furthermore, as the current density increased to 30 A g⁻¹, the capacitance could maintain 58% of the initial value. Thus, it can be concluded that the abandoned catalysts may serve as a potential electrode material for energy storage; simultaneously, the reutilization could eliminate secondary pollution and decrease the energy consumption in efficiency.

A piezoelectric catalyst of the MoS₂@TNr composite (MoS₂ nanosheets composited with TiO₂ nanorods) was synthesized by a two-step hydrothermal method, and can be recycled and reused as an advanced anode material for supercapacitors.     

Hierarchical Co3O4@Co9S8 nanowall structures assembled by many nanosheets for high performance asymmetric supercapacitors

Herein, we report hierarchical Co₃O₄@Co₉S₈ nanowalls assembled by many nanosheets. The as-synthesized products are characterized in detail using various characterization methods. They can be directly used as supercapacitor electrodes with excellent electrochemical performance due to the synergy effect between Co₃O₄ and Co₉S₈. Furthermore, a flexible asymmetric supercapacitor is fabricated by using the as-synthesized Co₃O₄@Co₉S₈ structures as the cathode and the active carbon as the anode, which reveals a specific capacitance of 266.6 mF cm⁻² at a current density of 4 mA cm⁻². In addition, the supercapacitor shows an excellent capacity retention rate of 86.5% after 10 000 cycles at a current density of 10 mA cm⁻². Finally, three supercapacitor devices connected in series can light a blue LED lamp for 5 min.

Herein, we report hierarchical Co₃O₄@Co₉S₈ nanowalls assembled by many nanosheets.     

Hydrogen-etched CoS2 to produce a Co9S8@CoS2 heterostructure electrocatalyst for highly efficient oxygen evolution reaction

There is a pressing requirement for developing high-efficiency non-noble metal electrocatalysts in oxygen evolution reactions (OER), where transition metal sulfides are considered to be promising electrocatalysts for the OER in alkaline medium. Herein, we report the outstanding OER performance of Co₉S₈@CoS₂ heterojunctions synthesized by hydrogen etched CoS₂, where the optimized heterojunction shows a low η₅₀ of 396 mV and a small Tafel slope of 181.61 mV dec⁻¹. The excellent electrocatalytic performance of this heterostructure is attributed to the interface electronic effect. Importantly, the post-stage characterization results indicate that the Co₉S₈@CoS₂ heterostructure exhibits a dynamic reconfiguration during the OER with the formation of CoOOH in situ, and thus exhibits a superior electrocatalytic performance.

Herein, we report the outstanding OER performance of Co₉S₈@CoS₂ heterojunctions synthesized by hydrogen etched CoS₂, where the optimized heterojunction shows a low η₅₀ of 396 mV and a small Tafel slope of 181.61 mV dec⁻¹.