3D hierarchical rose-like Ni2P@rGO assembled from interconnected nanoflakes as anode for lithium ion batteries

In recent years, anode materials of transition metal phosphates (TMPs) for lithium ion batteries (LIBs) have drawn a vast amount of attention from researchers, due to their high theoretical capacity and comparatively low intercalation potentials vs. Li/Li⁺. However, in practice, their application remains constrained by poor electrical conductivity, and dramatic volume expansion and severe agglomeration during the lithium process, which leads to questionable kinetic issues and a prompt decline in capacity during cycling. Herein, through an elaborate design, we developed a novel three-dimensional (3D) hierarchical rose-like architecture self-assembled from two-dimensional (2D) Ni₂P nanoflakes immobilized on reduced graphene oxide (rGO) via a combination of a hydrothermal process and phosphating treatment. Such a design provides unique superiority for Ni₂P-based anode materials for LIBs. Paraphrasing, the 3D hierarchical structure of Ni₂P distributes the stress on the anode material while cycling and provides more lithium storage space. The rGO not only enhances the conductivity of materials, but also serves as a flexible framework which immobilizes Ni₂P so that it prevents it from pulverization. Therefore, the synergistic effect between them guarantees the integrity of the material structure after a long-term cycling Li⁺ intercalation and deintercalation process. When it acted as anode material for LIBs, the as-obtained 3D rose-like Ni₂P@rGO electrode exhibited a noticeable electrochemical performance, which delivers a discharge capacity of 330.5 mA h g⁻¹ at a current density of 100 mA g⁻¹ after 100 cycles and retains 200.5 mA h g⁻¹ at 1000 mA g⁻¹.

In recent years, anode materials of transition metal phosphates (TMPs) for lithium ion batteries have drawn a vast amount of attention, due to their high theoretical capacity and comparatively low intercalation potentials vs. Li/Li⁺.     

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.     

Investigation of Cu doped flake-NiO as an anode material for lithium ion batteries

Cu foil is widely used in commercial lithium ion batteries as the current collector of anode materials with excellent conductivity and stability. In this research, commercial Cu foil was chosen as the current collector and substrate for the synthesis of Cu doped flake-NiO via a traditional hydrothermal method. The effect of the ratio of Cu and the calcination temperature on the electrochemical performance of NiO was investigated. The structure and phase composition of the Cu doped flake-NiO electrode were studied through X-ray diffraction (XRD), scanning electron microscopy (SEM), Energy dispersive X-ray analysis (EDAX), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and inductive coupled plasma emission spectrometry (ICP). The electrochemical properties of the Cu doped flake-NiO electrode were studied through cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and a galvanostatic charge–discharge cycling technique. According to the results, the Cu-doped NiO electrode, calcined at 400 °C with a molar ratio of Cu : Ni = 1 : 8, exhibited a high reversible charge capacity. The good cycling stability and rate performance indicate that the as-prepared electrode can be applied as a potential anode for lithium ion batteries.

Cu doped flake-NiO shows excellent electrochemical performance as anode materials for lithium ion batteries.     

Sonochemistry-enabled uniform coupling of SnO2 nanocrystals with graphene sheets as anode materials for lithium-ion batteries

SnO₂/graphene nanocomposite was successfully synthesized by a facile sonochemical method from SnCl₂ and graphene oxide (GO) precursors. In the sonochemical process, the Sn²⁺ is firstly dispersed homogeneously on the GO surface, then in situ oxidized to SnO₂ nanoparticles on both sides of the graphene nanosheets (RGO) obtained by the reduction of GO under continuous ultrasonication. Graphene not only provides a mechanical support to alleviate the volume changes of the SnO₂ anode and prevent nanoparticle agglomeration, but also serves as a conductive network to facilitate charge transfer and Li⁺ diffusion. When used as a lithium ion battery (LIB) anode, the SnO₂/graphene nanocomposite exhibits significantly improved specific capacity (1610 mA h g⁻¹ at 100 mA g⁻¹), good cycling stability (retaining 87% after 100 cycles), and competitive rate performance (273 mA h g⁻¹ at 500 mA g⁻¹) compared to those of bare SnO₂. This sonochemical method can be also applied to the synthesis of other metal-oxide/graphene composites and this work provides a large-scale preparation route for the practical application of SnO₂ in lithium ion batteries.

SnO₂/graphene nanocomposite was successfully synthesized by a facile sonochemical method from SnCl₂ and graphene oxide (GO) precursors.     

Facile in situ growth of ZnO nanosheets standing on Ni foam as binder-free anodes for lithium ion batteries

ZnO has attracted increasing attention as an anode for lithium ion batteries. However, the application of such anode materials remains restricted by their poor conductivity and large volume changes during the charge/discharge process. Herein, we report a simple hydrothermal method to synthesize ZnO nanosheets with a large surface area standing on a Ni foam framework, which is applied as a binder-free anode for lithium ion batteries. ZnO nanosheets were grown in situ on Ni foam, resulting in enhanced conductivity and enough space to buffer the volume changes of the battery. The ZnO nanosheets@Ni foam anode showed a high specific capacity (1507 mA h g⁻¹ at 0.2 A g⁻¹), good capacity retention (1292 mA h g⁻¹ after 45 cycles), and superior rate capacity, which are better than those of ZnO nanomaterial-based anodes reported previously. Moreover, other transition metal oxides, such as Fe₂O₃ and NiO were also formed in situ on Ni foam with perfect standing nanosheets structures by this hydrothermal method, confirming the universality and efficiency of this synthetic route.

ZnO nanosheets@Ni foam anode showed a high specific capacity, good capacity retention and superior rate capacity. Moreover, other transition metal oxides were also similarly formed on Ni foam, confirming the universality and efficiency of the synthetic route.     

Design and synthesis of graphene/SnO2/polyacrylamide nanocomposites as anode material for lithium-ion batteries

Tin dioxide (SnO₂) is a promising anode material for lithium-ion batteries owing to its large theoretical capacity (1494 mA h g⁻¹). However, its practical application is hindered by these problems: the low conductivity, which restricts rate performance of the electrode, and the drastic volume change (400%). In this study, we designed a novel polyacrylamide/SnO₂ nanocrystals/graphene gel (PAAm@SnO₂NC@GG) structure, in which SnO₂ nanocrystals anchored in three-dimensional graphene gel network and the polyacrylamide layers could effectively prevent the agglomeration of SnO₂ nanocrystals, presenting excellent cyclability and rate performance. A capacity retention of over 90% after 300 cycles of 376 mA h g⁻¹ was achieved at a current density of 5 A g⁻¹. In addition, a stable capacity of about 989 mA h g⁻¹ at lower current density of 0.2 A g⁻¹ was achieved.

Tin dioxide (SnO₂) is a promising anode material for lithium-ion batteries owing to its large theoretical capacity (1494 mA h g⁻¹).     

Three-dimensional clusters of peony-shaped CuO nanosheets as a high-rate anode for Li-ion batteries

Copper oxide (CuO) has emerged as a promising anode material for lithium-ion batteries (LIBs) due to its high theoretical capacity, low cost and low toxicity to the natural environment. However, the low electronic/ionic conductivity and the imponderable volume expansion problem during the charge and discharge process hinder its practical applications. Herein, we introduced three-dimensional clusters of peony-shaped CuO nanosheets (TCP-CuO) to tackle these problems by increasing the surface of the active material. The unique three-dimensional peony-liked structure not only alleviates the volume expansion problem, but also enhances the electron/ion conductivity. As a result, the TCP-CuO electrode possesses a reversible high capacity of 641.1 mA h g⁻¹ at 0.1 A g⁻¹ after 80 cycles as well as an outstanding rate performance of 531.6 mA h g⁻¹ at a rate of 5C (1C = 674 mA g⁻¹) and excellent cycling durability of 441.1 mA h g⁻¹ at 1 A g⁻¹ after 100 cycles. This work demonstrated a novel strategy for the construction of a well-designed structure for other advanced energy-storage technologies.

TCP-CuO possesses a unique three-dimensional peony-liked architecture which not only alleviates volume expansion problem, but also enhances electron/ion conductivity. As an anode in LIBs, TCP-CuO delivers excellent electrochemical performances.     

Synthesis of nano-sized urchin-shaped LiFePO4 for lithium ion batteries

In this article, the facile synthesis of sea urchin-shaped LiFePO₄ nanoparticles by thermal decomposition of metal-surfactant complexes and application of these nanoparticles as a cathode in lithium ion secondary batteries is demonstrated. The advantages of this work are a facile method to synthesize interesting LiFePO₄ nanostructures and its synthetic mechanism. Accordingly, the morphology of LiFePO₄ particles could be regulated by the injection of oleylamine, with other surfactants and phosphoric acid. This injection step was critical to tailor the morphology of LiFePO₄ particles, converting them from nanosphere shapes to diverse types of urchin-shaped nanoparticles. Electron microscopy analysis showed that the overall dimension of the urchin-shaped LiFePO₄ particles varied from 300 nm to 2 μm. A closer observation revealed that numerous thin nanorods ranging from 5 to 20 nm in diameter were attached to the nanoparticles. The hierarchical nanostructure of these urchin-shaped LiFePO₄ particles mitigated the low tap density problem. In addition, the nanorods less than 20 nm attached to the edge of urchin-shaped nanoparticles significantly increased the pathways for electronic transport.

In this article, the facile synthesis of sea urchin-shaped LiFePO₄ nanoparticles by thermal decomposition of metal-surfactant complexes and application of these nanoparticles as a cathode in lithium ion secondary batteries is demonstrated.     

Amorphous red phosphorus incorporated with pyrolyzed bacterial cellulose as a free-standing anode for high-performance lithium ion batteries

Amorphous red phosphorus/pyrolyzed bacterial cellulose (P-PBC) free-standing films are prepared by thermal carbonization and a subsequent vaporization-condensation process. The distinctive bundle-like structure of the flexible pyrolyzed bacterial cellulose (PBC) matrix not only provides sufficient volume to accommodate amorphous red-phosphorus (P) but also restricts the pulverization of red-P during the alternate lithiation/delithiation process. When the mass ratio of raw materials, red-P to PBC, is 70 : 1, the free-standing P-PBC film anode exhibits high reversible capacity based on the mass of the P-PBC film (1039.7 mA h g⁻¹ after 100 cycle at 0.1C, 1C = 2600 mA g⁻¹) and good cycling stability at high current density (capacity retention of 82.84% after 1000 cycles at 2C), indicating its superior electrochemical performances.

A novel freestanding anode was prepared by combining amorphous red-P with a pyrolyzed bacterial cellulose (PBC) matrix for the first time.     

Rheological phase reaction synthesis and electrochemical performance of rufigallol anode for lithium ion batteries

1,2,3,5,6,7-hexahydroxy-anthraquinone (rufigallol) and its metal–organic complex (rufigallol-Li/Ni, R-LN) were both synthesized. The electrochemical performance investigation of rufigallol and R-LN as anodes for lithium ion batteries indicates that pure rufigallol delivers high initial capacity but poor cycling stability, by contrast, the R-LN complex exhibits high initial capacity and excellent cycling stability.

Rufigallol was synthesized by rheological phase method with high yield and investigated as anode for lithium ion batteries.

Lithium ion batteries (LIBs) have proven to be clean and efficient energy-storage technologies to meet the growing demand for green and sustainable electric power storage. Considering the sustainability, low cost, abundant natural sources, structural design with tolerance for variable functional groups, fast reaction kinetics and high power density, organic electrode materials have been the most competitive alternative to traditional inorganic materials.^1–6 Quinone organic materials with carbonyl functional group have received great concern for their high theoretical capacities and reaction reversibility. While, the dissolution of small molecule quinones in the organic electrolyte and the resulting capacity attenuation have restricted their application. Therefore, it is urgently demanded to design new organic electrode materials with high energy efficiency and good cycling stability. Among small molecule quinones, 1,2,3,5,6,7-hexahydroxy-anthraquinone (rufigallol) as a critical component has been widely used in dye industry, drug synthesis, and organic materials.^7–10 Rufigallol was firstly synthesized in 1836.¹¹ Since then, very little efficient method for the preparation of rufigallol has been reported. Recently, Bisoyi and Kumar''s research indicates that the rufigallol can be achieved by self-condensation of gallic acid in the presence of sulfuric acid under microwave-assisted, and the yield reached 86%.¹² Also, as far as we know, there is no correlative literature exists about rufigallol being used as electrode materials for lithium ion batteries.It is well-known that the dissolution of organic molecule in the electrolyte can be effectively suppressed via salt formation. Therefore, constructing the metal–organic complex is an efficient way to obtain a stable and flexible framework as well as a better cycling stability.^13–19 In our previous study,^20,21 we have successfully realized the improvement of cycling stability for 3,4,9,10-perylene-tetracarboxylic acid-dianhydride (NTCDA) through the introducing of Li/Ni or Co/Mn to the matrix material. The obtained metal–organic complex, namely, Li/Ni-1,4,5,8-naphthalenetetracarboxylate or Co/Mn-1,4,5,8-naphthalenetetracarboxylate, showed a high specific capacity and a good cycling stability.Herein, we developed a simple, economical and effective rheological phase method²² to synthesize the rufigallol with high yield and investigated its electrochemical application in lithium ion batteries. In view of the easily connecting of hydroxyl groups in aromatic carbonyl compound for rufigallol with metal ions, in order to modify the cycling stability of pure rufigallol, we synthesized rufigallol-Li/Ni complex (R-LN) through the introducing of lithium and nickel by a hydrothermal method. When used as the LIBs anode, rufigallol shows initial discharge and charge capacities of 977 mA h g⁻¹ and 460 mA h g⁻¹, respectively, which is far higher than that of traditional graphite anode. The R-LN complex delivers initial a charge capacity of 560 mA h g⁻¹ and still remain at about 500 mA h g⁻¹ after 100 cycles, indicating a good electrochemical performance. Our work presents a new quinone-based organic materials with high capacity and competitive cycling stability, which enriches the organic electrode materials.     

Application of soot discharged from the combustion of marine gas oil as an anode material for lithium ion batteries

Many studies have recently investigated the characteristics of combustion products emitted from ships and onshore plant facilities for use as energy sources. Most combustion products that have been reported until now are from heavy oils, however, no studies on those from light oils have been published. This study attempted to use the combustion products from the light oils from naval ships as anode materials for lithium ion batteries (LIBs). These products have a carbon black morphology and were transformed into highly crystalline carbon structures through a simple heat treatment. These new structured materials showed reversible capacities of 544, 538, 510, 485, 451 and 395 mA h g⁻¹ at C-rates of 0.1, 0.2, 0.5, 1.0, 2.0 and 5.0C, respectively, and excellent rate performance. These findings were the result of a combination hierarchical pores ranging from the meso- to macroscale and the high capacitive charge storage behavior of the soot. The results of this study prove that annealed soot with a unique multilayer graphite structure shows promising electrochemical performance suitable for the production of low-cost, high-performance LIB anode materials.

Many studies have recently investigated the characteristics of combustion products emitted from ships and onshore plant facilities for use as energy sources.     

Heterogeneous synthesis and electrochemical performance of LiMnPO4/C composites as cathode materials of lithium ion batteries

In this study, a facile yet efficient interfacial hydrothermal process was successfully developed to fabricate LiMnPO₄/C composites. In this strategy, the walls of carbon nanotubes were employed as heterogeneous nucleation interfaces and biomass of phytic acid (PA) as an eco-friendly phosphorus source. By comparing the experimental results, a reasonable nucleation-growth mechanism was proposed, suggesting the advantages of interfacial effects. Meanwhile, the as-synthesized LiMnPO₄/C samples exhibited superior rate performances with discharge capacities reaching 161 mA h g⁻¹ at C/20, 134 mA h g⁻¹ at 1C, and 100 mA h g⁻¹ at 5C. The composites also displayed excellent cycling stabilities by maintaining 95% of the initial capacity over 100 continuous cycles at 1C. Electrochemical impedance spectroscopy showed that the superior electrochemical performances were attributed to the low charge-transfer resistance and elevated diffusion coefficient of lithium ions. In sum, the proposed approach for the preparation of LiMnPO₄/C composites looks promising for future production of composite electrode materials for high-performance lithium-ion batteries.

A heterogeneous nucleation technique was used to prepare LiMnPO₄/C. The walls of carbon nanotubes were employed as nucleation interfaces and phytic acid as an eco-friendly phosphorus source. The product exhibits superior electrochemical performance.     

Rationally designed hierarchical porous CNFs/Co3O4 nanofiber-based anode for realizing high lithium ion storage

To achieve a high power density of lithium-ion batteries, it is essential to develop anode materials with high capacity and excellent stability. Cobalt oxide (Co₃O₄) is a prospective anode material on account of its high energy density. However, the poor electrical conductivity and volumetric changes of the active material induce a dramatic decrease in capacity during cycling. Herein, a hierarchical porous hybrid nanofiber of ZIF-derived Co₃O₄ and continuous carbon nanofibers (CNFs) is rationally constructed and utilized as an anode material for lithium-ion batteries. The PAN/ZIF-67 heterostructure composite nanofibers were first synthesized using electrospinning technology followed by the in situ growth method, and then the CNFs/Co₃O₄ nanofibers were obtained by subsequent multi-step thermal treatment. The continuous porous conductive carbon backbone not only effectively provides a channel to expedite lithium ion diffusion and electrode transfer, but also accommodates volume change of Co₃O₄ during the charge–discharge cycling process. The electrode exhibits a high discharge capacity of 1352 mA h g⁻¹ after 500 cycles at a constant current density of 0.2 A g⁻¹. Additionally, the composites deliver a discharge capacity of 661 mA h g⁻¹ with a small capacity decay of 0.078% per cycle at a high current density of 2 A g⁻¹ after 500 cycles. This hierarchical porous structural design presents an effective strategy to develop a hybrid nanofiber for improving lithium ion storage.

Hierarchical porous CNFs/Co₃O₄ nanofiber is rationally designed and constructed as an anode for achieving high capacity and stable lithium ion batteries.     

Three-dimensional hierarchical ZnCo2O4@C3N4-B nanoflowers as high-performance anode materials for lithium-ion batteries

ZnCo₂O₄ has become one of the most widely used anode materials due to its good specific capacity, cost-efficiency, high thermal stability and environmental benignity. However, its poor conductivity and cycle stability have limited its practical application in lithium-ion batteries. To overcome these issues, we constructed a 3D nanoflower composite material (ZnCo₂O₄@C₃N₄-B) by combining ZnCo₂O₄ as a framework and B-doped g-C₃N₄ (g-C₃N₄-B) as a new carbon source material via a simple hydrothermal method. ZnCo₂O₄@C₃N₄-B exhibited exceptional specific capacitance of 919.76 mA h g⁻¹ after 500 cycles at 0.2 A g⁻¹ and a long-term capacity retention of 97.8% after 1000 cycles at 2 A g⁻¹. The high reversible capacity, long cycling life and good rate performance could be attributed to the 3D interconnected architecture and doping of g-C₃N₄-B. This work provides a simple and general strategy to design high-performance anode materials for lithium-ion batteries to meet the needs of practical applications.

ZnCo₂O₄ has become one of the most widely used anode materials due to its good specific capacity, cost-efficiency, high thermal stability and environmental benignity.     

Hard-template-engaged formation of Co2V2O7 hollow prisms for lithium ion batteries

Highly uniform and monodispersed Co₂V₂O₇ hollow nanoprisms are successfully fabricated via a simple hard-template-engaged strategy. In such synthesis, Co acetate hydroxide plays the important role of not only the cobalt source, but also the sacrificial template. After coating with a carbon layer, the as-obtained Co₂V₂O₇ hollow nanoprisms exhibit apparently enhanced lithium-ion-battery performances. It has shown a high specific capacity of around 946 mA h g⁻¹ at a current density of 100 mA g⁻¹, excellent cycling stability up to 300 cycles at a current density of 1 A g⁻¹, and superior rate performance, which is much better than carbon coated solid Co₂V₂O₇ samples.

A novel hard-template-assisted method is developed to fabricate high quality Co₂V₂O₇ hollow nanoprisms. When evaluated as anode materials for lithium ion batteries, they exhibited a remarkable enhanced performance compared with the solid ones.

In the past decades, research on clean and environment-friendly energy resources, which are considered as the most efficient way to solve the increasingly serious energy crisis, has received continuous attention.^1–3 Among the varieties of solutions, lithium ion batteries (LIB) are some of the most important energy storage devices due to their various merits of high voltage, high recharge capability, low cost, and environmental friendliness.^4–7 However, there are still several challenging issues in the actual research and application, such as the low power density, relatively low capacity compared with the theoretical value as well as the problems of longer cycle life and better rate performance. Thus, developing new kinds of LIB materials, including component controlling, shape engineering and structure designing, is still the most important and urgent target to be achieved.^8,9Previous reports have firmly confirmed that inorganic nanoparticles (NPs) with hollow inter structures always exhibit highly increased LIBs performances compared with the solid ones,^10–15 owning to their larger specific surface area, good accessibility, shorter distance over which lithium ions must diffuse in the solid state, and more importantly, the larger increased structural stability during the charge–discharge process. Among the variety kinds of available methods to prepare hollow structured materials, hard-template engaged strategy has become the research hot-spot very recently. Such synthesis involves the use of various active or inactive particles to support the deposition/growth of shell materials, followed by etching treatment to remove the template components. Compared with the template-free method, hard-template method has some important advantages: (1) the usage of hard template is more suitable to realize the precise timing of inner hollow space, fine structure of outer shell and even the whole morphology; (2) uniform hollow space is much easier to be fabricated; (3) it is an efficient way to couple multiple components together, that increasing the complexity of hollow structures in a designed manner may bring more possibilities in modulating the properties of functional nanostructures for many applications. In generally speaking, an idea hard template should have good activity with designed shell component and also be easily removed after the synthesis. With the rapid development of synthetic nanotechnology, many kinds of hollow NPs have been successfully prepared.^16–33 For examples, using Cu₂O sphere, cubic or octahedral particles as sacrificial templates, a series of metal oxide-based spherical or non-spherical nanocages can be prepared;^23–26 Porous carbon or carbon/metal polyhedrons have been synthesized by directly annealing metal–organic-frameworks (MOFs) as hard templates.^16–22 Otherwise, MnCO₄ nanocubes,²⁷ Cu nanowires,²⁸ ZnSnO₃ nanocubes,²⁹ graphene nanosheets³⁰ and et al. are also used as the hard templates to construct functional hollow nanomaterials. Very recently, a new kind of hard template, metal-Ac has been successfully developed to assist the formation of hollow NPs. As reported by Lou''s group, metal-Ac is unstable. The precipitation can self-decomposed completely in aqueous solution, indicating the high activity. By utilizing such structural feature, a series of Co-based mixed metal oxides and sulfides are constructed,^31–33 with excellent performances in energy-related areas.Cobalt-based mixed metal oxides with different metal cations have demonstrated high electrochemical activities due to their interfacial effects and the synergic effects of the multiple metal species. For instance, cobalt vanadate is one of the most important materials for energy storage.^34–37 That is because introducing vanadium to couple with cobalt to form binary cobalt vanadates can largely enhance the electronic/ionic conductivity, reversible capacity and mechanical stability compared with single-component cobalt oxides. Many kinds of cobalt vanadate NPs with tunable components and shapes been reported, including self-assembled Co₃V₂O₈ nanosheets,³⁴ Co₃V₂O₈ sponge networks³⁵ and Co₂V₂O₇–graphene hybrid nanocomposites.³⁶ However, to the best of our knowledge, hollow cobalt vanadates is rarely reported, except the current reported Co₃V₂O₈·nH₂O hollow hexagonal prismatic pencils with micron in diameters by Yu and co-workers synthesized via a hydrothermal approach.³⁷ The challenges still exist in decreasing the particle sizes of cobalt vanadate and controlling the interior structures, as well as the compositions. Thus, it would be of great interest to develop a facile approach to synthesize novel cobalt vanadate with controllable hollow nanostructure.Inspired by the previous reports, herein, we present a facile template-engaged approach for the synthesis of Co₂V₂O₇ hollow prisms with controllable morphology and inner nanostructure via an ion-replacement reaction between Co acetate hydroxide and Na₃VO₄. Co acetate hydroxide plays the important role of not only the cobalt source, but also the sacrificed template. Impressively, after surface coating with thin carbon layer, the as-obtained hollow prisms manifest higher specific capacitance with enhanced cycling stability compared with the solid ones, making them potential electrode materials for LIBs.The whole synthesis is summarized in the Scheme 1 (details are shown in the ESI^†). In the first step, Co acetate hydroxide precursors were prepared as described by Lou''s group.^31,32 The morphology of the precursor is revealed by TEM as depicted in Fig. S1.^† The result matches well with the previous report, that the precursors have uniform contrast, clearly indicating their solid and dense nature. The as-obtained 1-D nanoprisms are around 450 nm in length and 75 nm in width. Fig. S2^† is the XRD data, which could be assigned to a tetragonal Co₅(OH)₂(CH₃COO)₈(H₂O)₂ phase.Open in a separate windowScheme 1Schematic illustration of the synthesis of hollow Co₂V₂O₇ sample.Next, we chose Na₃VO₄ as the V source to reacted with Co acetate hydroxide owning to its high solubility in water. Generally, V-based raw materials always have poor aqueous solubility. For example, NH₃VO₃ is a common one, but it is hard to be dissolved in water unless excess H⁺ or OH⁻ ions are existed. However, the presence of free H⁺ or OH⁻ ions can strongly accelerate the dissolution or hydrolysis of Co acetate hydroxide, which is fatal for the following ion-exchange reaction. In addition, there are another two key points in the synthetic steps to avoid the self-decomposition of Co acetate hydroxide precursors: first, it is necessary to use absolute ethanol to pre-disperse Co-based precursors; the next one is the fast injection of Na₃VO₄ aqueous solution into the reaction system. With the typical photos shown in Fig. S3,^† after adding Na₃VO₄ aqueous solution, the original pink solution has become to ochre colour within 20 seconds, clearly indicating the successful formation of ion-exchange reaction between Co acetate hydroxide and Na₃VO₄. A series of characterizations have been used to obtain the structural information of the product. With the data displayed in Fig. 1, the corresponding TEM images have shown that the product well maintained the original 1D prism-like nanostructure with slightly shrunken length (around 400 nm) and expended width (around 100 nm). Compared with the original smooth surface, the surface of the as-formed nanoprisms kept well. Interestingly, well-defined inner cavities could be found in every nanoprism, which could be easily distinguished by the sharp contrast between the center and the edge. Furthermore, the HAADF-STEM-Mapping technique was also used for the analysis of the fine distribution of the Co and V elements in the nanostructures. As shown in the image presented in Fig. 1d–f, both of the two elements spread throughout the hollow nanoprisms uniformly. The atomic ratio of Co and V is 1 : 1.07, which can also be calculated from the EDX data (with the data shown in Fig. S4^†). The XRD pattern of the as-obtained product was also tested and is shown in Fig. 1c. No clear diffraction peaks could be found, indicating the product is in the form of amorphous phase.Open in a separate windowFig. 1(a) and (b): TEM images of the as-obtained amorphous Co–V mixed metal oxides; (c) the corresponding XRD data; (d) to (f) HAADF-STEM-Mapping analysis of the Co–V mixed metal oxides; (g) and (h) TEM images of the Co₂V₂O₇ obtained via a heating treatment; (i) TEM image of Co₂V₂O₇@C core@shell composite.A series of control experiments was designed to gain a deeper insight into the formation mechanism. Initially, we attempted to use water as solvent to instead of ethanol to disperse Co acetate hydroxide precursors. It is found that the as-obtained Co-based precursors are easily dissolved in water within a few minutes completely. Further using such clear solution to react with VO₄³⁻ couldn''t obtain ordered product, only irregular small sized nanoparticles are found from the TEM images shown in Fig. S5.^† It could be understood that without the assistant of hard-template, the product is hard to keep uniform morphology. Next, we performed the synthesis under the same conditions, except at higher temperature (80 °C). Fig. S6^† is the corresponding TEM result. The quality of the product is poor, and many broken areas are also found. But the hollow inner nanostructures are still well maintained. The possible reason could be attributed to the increased hydrolyzing trend of Co acetate hydroxide at higher temperature. Finally, we also investigate the effect of Na₃VO₄ feeding amount towards the final product by doubling the Na₃VO₄ amount. As shown in Fig. S7,^† no obvious change could be found. The product is also high-quality and in the form of prism-like hollow nanostructure. Further ICP analysis is also been taken. The result confirmed that the molar ratio of Co and V is also around 1/1, indicating that such stoichiometric ratio is thermodynamic stable.By referring to the above experiments, it is considered that the whole synthesis is a typical speed race process involving two independent reactions after addition of Na₃VO₄ aqueous solution: the hydrolysis of Co acetate hydroxide and the ion-replacement reaction between Co acetate hydroxide and Na₃VO₄. Unfortunately, we can not find the exact data about the solubility product constant (K_sp) of Co acetate hydroxide as well as cobalt vanadate complex. But we noticed that Co acetate hydroxide can self-decompose in pure water but cobalt vanadate is stable enough, indicating the K_sp (cobalt vanadate) > K_sp (Co acetate hydroxide). Thus the ion-replacement reaction is driven by thermodynamics. Additional, our synthetic result has also shown that the binding energy between Co-(VO₄) is much stronger than Co-(OH). As a result, Co–V mixed metal oxide is formed and Co–OH is allowable in thermodynamics. Further referring to the previous report, it is considered that a thin Co–V mixed metal oxide layer is in situ formed on the outer surface of Co acetate hydroxide precursor at the start of the reaction. Here, the as-generated Co–V mixed metal oxide acts as a physical barrier to hinder a direct chemical reaction between outside (VO₄³⁻) ions and inner metal-ion species. Therefore, further reaction depends on the relative diffusion of metal or VO₄³⁻ ions through this newly formed oxide layer. Along with the reaction going, the outward flow of Co²⁺ will consume the core template, leading to the generation of void space within the Co–V mixed metal oxide shells. Thus, the hollow nanostructure is formed.Finally, the pre-synthesized amorphous Co–V mixed metal oxides are further annealed at 350 °C in air to accomplish the formation of crystallized phase. The corresponding XRD patterns are shown in Fig. S8,^† the peaks between 2θ = 23.2° and 40.1° indicates the formation of well crystallized Co₂V₂O₇ phase (JCPDS card no. 01-070-1189, space group P21/n). No additional diffraction peaks could be observed from the XRD patterns, suggesting the high purity of the products. From the low-magnification TEM image in Fig. 1g, it is clear the as-obtained products are uniform and monodisperse. The enlarged TEM image in Fig. 1h confirms the particle size and the inner hollow space of the annealed sample are kept well compared with the un-heated ones, indicating the unique structural stability of the hollow nanostructure. The only difference is the original smooth surface becomes much rougher. The thickness of the coated carbon layer is about 10 nm, which is obtained from the corresponding TEM images shown in Fig. 1i and inset. X-ray photoelectron spectroscopy (XPS) analysis is carried out to characterize the chemical state of the elements in Co₂V₂O₇ hollow nanoprisms. As shown in Fig. S9,^† the two main peaks at 786.5 eV and 803.0 eV that can be assigned to the Co 2p3/2 and 2p1/2 spin orbit peaks, respectively, whereas the peaks at 516.8 and 517.5 eV correspond well to the V 2p3/2 and 2p1/2 spin orbit, which firmly confirmed that the valence state of Co and V is +2 and +5, respectively. Additional, the Co and V contents were 35 and 33 wt%, respectively, as determined by elemental analysis using inductively coupled plasma atomic emission spectrometry (ICP-AES), that matches well with the EDX data of unheated sample. The specific surface area of the as-obtained Co₂V₂O₇ hollow nanoprisms calculated from the BET curve (Fig. S10^†) reaches approximately 97 m² g⁻¹. Such a large BET surface is possibly caused by the inner hollow structures.The complexity of the hollow nanoprisms in both structure and composition might endow the materials with exceptional properties and performance in particular applications. In this work, we further demonstrate the remarkable lithium storage properties of the hollow Co₂V₂O₇ nanoprisms, which are associated with their unique nanostructure and high complexity. Otherwise, solid Co₂V₂O₇ nanoparticles and hollow Co₃O₄ nanoprisms are also synthesized and used as the references (details are shown in the ESI^†). Before the LIB test, the three kinds of anode materials are further coated with carbon to further enhance (with the typical TEM images of carbon coated hollow Co₂V₂O₇ nanoprisms shown in Fig. 1i and inset) their stabilities (experiment details and corresponding TEM images are shown in ESI^†). Fig. S11^† depicts the cyclic voltammogram (CV) profile obtained at a scan rate of 0.2 mV s⁻¹ in the potential window of 3 V to 0.01 V. In the first cathodic sweep or during the lithiation step, two reduction peaks can be clearly identified at around 0.45, and 0.05 V, corresponds to the electrochemical reduction of Co₂V₂O₇ to CoO accompanied by the lithiation of V₂O₅, further reduction into metallic Co and Li_1+xVO₂, respectively. Apparently, the peak intensity drops significantly in the second cycle, indicating the occurrence of some irreversible electrochemical processes in the first cycle. On the other hand, the anodic sweep of the first cycle shows two distinguished oxidation peaks at 1.29 and 2.36 V, which are ascribed to the formation of CoO from metallic Co and the delithiation of the vanadium oxides.The discharge voltage profiles of the three carbon-coated samples at a current density of 100 mA g⁻¹ are shown in Fig. 2a. For carbon coated Co₂V₂O₇ nanoprisms, a distinct voltage plateau at ca. 1.21 V was clearly identified during the initial discharge process. The peak can be assigned to the transformation of Co₂V₂O₇ into CoO accompanied by the formation of Li_xV₂O₅ and the reduction of Co²⁺ to Co, which is in agreement with results reported in the literature.^34,37 The rate capabilities of the three samples are investigated by gradually increasing the current density from 0.1 to 5 A g⁻¹ and then returning it to 0.1 A g⁻¹ (Fig. 2b). The average discharge capacities of the carbon coated Co₂V₂O₇ hollow nanoprisms are 908, 876, 848, 732, 632 and 465 mA h g⁻¹ at the current densities of 0.1, 0.2, 0.5, 1, 2 and 5 A g⁻¹, respectively. Interestingly, when the current density was returned to 0.1 A g⁻¹ after the 60 cycles, the specific capacity could be well recovered to 946 mA h g⁻¹, which is even much better than the initiative ten cycles at 0.1 A g⁻¹. For comparison, despite the carbon coated Co₃O₄ hollow nanoprisms and Co₂V₂O₇ solid NPs also exhibit excellent rate and cycle capabilities, their specific capacities at all the current densities are much lower than carbon coated Co₂V₂O₇ hollow nanoprisms. The values are 639, 588, 523, 472, 369, 267 mA h g⁻¹ (for carbon coated Co₃O₄ hollow nanoprisms) and 770.8, 703.3, 647.5, 589.4, 547.6, 347.6, 766.3 mA h g⁻¹ (for carbon coated Co₂V₂O₇ solid NPs) at the current densities of 0.1, 0.2, 0.5, 1, 2 and 5 A g⁻¹, respectively. The differences of the rate capacities between Co₂V₂O₇ hollow nanoprisms and solid nanoparticles could be attributed to the shape-effect. Obviously, the presence of hollow nanostructure is more efficient than the solid one for LIB applications.Open in a separate windowFig. 2Electrochemical performance of carbon coated Co₂V₂O₇ hollow nanoprisms (red line), carbon coated Co₃O₄ hollow nanoprisms (blue line) and Co₂V₂O₇ solid NPs (black line). (a) Discharge voltage profiles of the three samples at a current of 100 mA g⁻¹ in 1st cycle; (b) rate performance at different current densities; (c) cycling performance and corresponding CE of carbon coated Co₂V₂O₇ hollow nanoprisms at a current of 1 A g⁻¹.The cycling performance of Co₂V₂O₇ hollow nanoprism is depicted in Fig. 2C, at a constant current density of 1 A g⁻¹ between 0.01 and 3 V. It is interesting to observe that the discharge capacity decreases slightly before the 20th cycle. Beyond that point, there is a slight increase in the discharge capacity until the 300th cycle. Finally, the specific capacity could reach to 806 mA h g⁻¹. Such result is similar with the previously reported Co₃V₂O₈·H₂O sample.³⁷ The capacity fading at the first 20 cycles could be attributed to the deconstruction of crystal Co–V–O mixed metal oxides, which makes the reduction of the conductivity. After early cycles, the electrolyte can gradually penetrate into the inner part of the active materials, thus the specific capacity is increased. For better comparison, the cycling performance of bare Co₂V₂O₇ without the protection by carbon layer has also been tested. With the data shown in Fig. S12,^† Under the identical test conditions, the bare Co₂V₂O₇ hollow prisms exhibit much faster capacity fading, and a capacity of only around 393 mA h g⁻¹ is retained after 100 cycles. The result indicates the important role of carbon layer to help to maintain the hollow structure and improve the electrical conductivity.In all, highly uniform and monodisperse Co₂V₂O₇ nanoprisms with ordered inner hollow space have been successfully fabricated via a facile hard-template-engaged method. It is found that the coupling reaction between Co²⁺ and VO₄³⁻ has priority over the hydrolytic reaction of Co acetate hydroxide itself. Thus, the addition of Na₃VO₄ aqueous solution can cause an irreversible anion-exchanging reaction between Co acetate hydroxide and Na₃VO₄ driven by thermodynamics. After a facile annealing treatment in air, the amorphous Co–V–O mixed metal oxides can be transformed to crystal Co₂V₂O₇ nanoprisms with a well-maintained hollow interior. The as-synthesized Co₂V₂O₇ nanoprisms exhibit remarkable electrochemical performance as anode materials for high-performance LIBs by thin-carbon-layer coating. It is believed such “hard-template-assisted” strategy in the synthesis of mixed metal oxide with hollow interior is expected to be of great significance for the design and preparation of high efficient functional materials for clean energy.     

Electroless plating of a Sn–Ni/graphite sheet composite with improved cyclability as an anode material for lithium ion batteries

A Sn–Ni/graphite sheet composite is synthesized by a simple electroless plating method as an anode material for lithium ion batteries (LIBs). The microstructure and electrochemical properties of the composite are characterized by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), cyclic voltammetry (CV), and AC impedance spectroscopy. The results show that the as-prepared composite has Sn–Ni nanoparticles around 100 nm in size, where metallic Ni acts as an “anchor” to fix metallic Sn. The reunion phenomenon of Sn is alleviated by adding metallic Ni between the metallic Sn and graphite sheets. The Sn–Ni/graphite sheet electrode exhibits a good rate performance with a capability of 637.4, 586.3, 466.7, 371.5, 273.6, 165.3 and 97.3 mA h g⁻¹ at a current density of 0.1, 0.2, 0.5, 1.0, 2.0, 5.0 and 10 A g⁻¹, respectively. The good electrical conductivity of Ni, high specific capacity of Sn and excellent cycling capability of the graphite sheets have a synergistic effect and are the main reasons behind the superior electrochemical performance. Furthermore, the as-prepared composite exhibits excellent lithium storage capacity and the reversible capacity increased as the cycle number increased.

A Sn–Ni/graphite sheet composite is synthesized by a simple electroless plating method as an anode material for lithium ion batteries (LIBs).     

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.     

Plasma treated TiO2/C nanofibers as high performance anode materials for sodium-ion batteries

TiO₂ has been a promising anode material for sodium-ion batteries because it is low-cost and environment-friendly. However, its electrochemical performance at high rates is still not acceptable. Herein, we synthesized a TiO₂/C nanofiber material by the electrospinning method, and introduced air plasma treatments to modify the obtained material. Characterization results indicate that after the plasma treatments, the C fibers may have reacted with the plasma, and the surface areas of the nanofibers are increased. Electrochemical tests show this plasma treatment may be beneficial to the rate performance. The TiO₂/C nanofiber with plasma treatment could deliver a high redox capacity of 191 mA h g⁻¹ after 500 cycles at a very high rate of 10C (3300 mA g⁻¹). The superior effects of the plasma treatment on the rate performance may provide new insights for developing better materials for practical sodium-ion batteries.

Plasma treatment greatly improves the rate capability of TiO₂/C nanofibers.     

Rod-like anhydrous V2O5 assembled by tiny nanosheets as a high-performance cathode material for aqueous zinc-ion batteries

Aqueous zinc-ion batteries offer a low-cost and high-safety alternative for next-generation electrochemical energy storage, whereas suitable cathode materials remain to be explored. Herein, rod-like anhydrous V₂O₅ derived from a vanadium-based metal–organic framework is investigated. Interestingly, this material is assembled by tiny nanosheets with a large surface area of 218 m² g⁻¹ and high pore volume of 0.96 cm³ g⁻¹. Benefiting from morphological and structural merits, this material exhibits excellent performances, such as high reversible capacity (449.8 mA h g⁻¹ at 0.1 A g⁻¹), good rate capability (314.3 mA h g⁻¹ at 2 A g⁻¹), and great long-term cyclability (86.8% capacity retention after 2000 cycles at 2 A g⁻¹), which are significantly superior to the control sample. Such great performances are found to derive from high Zn²⁺ ion diffusion coefficient, large contribution of intercalation pseudocapacitance, and fast electrochemical kinetics. The ex situ measurements unveil that the intercalation of Zn²⁺ ion is accompanied by the reversible V⁵⁺ reduction and H₂O incorporation. This work discloses a direction for designing and fabricating high-performance cathode materials for zinc-ion batteries and other advanced energy storage systems.

V₂O₅ with intriguing micro/nano-hierarchical structure is fabricated via the pyrolysis of MIL-47 (a MOF material) and displays great performances as the cathode material for aqueous zinc-ion batteries.     

A porous silicon anode prepared by dealloying a Sr-modified Al–Si eutectic alloy for lithium ion batteries

Silicon has been considered to be one of the most promising anode materials for next generation lithium ion batteries due to its high theoretical specific capacity. However, its huge volume expansion during the lithiation/delithiation process that can result in rapid capacity fading and low conductivity present significant challenges for application. In this study, the morphology of Si in an Al–Si eutectic alloy was modified by Sr, and porous Si was then produced by dealloying the precursor. Profiting from the unique structure, the Si anode exhibits an excellent reversible capacity of 405 mA h g⁻¹ at 0.5 A g⁻¹ after 100 cycles and a fantastic first cycle coulombic efficiency of 83.74%. Furthermore, the porous silicon modified by Sr delivers a stable capacity of 594.8 mA h g⁻¹ even at a high current density of 2 A g⁻¹ after 50 cycles, suggesting a good rate capability.

With a porous coralloid structure, the silicon anode prepared by dealloying the Sr-modified Al–Si eutectic alloy exhibits excellent cycle and rate performances.