Facile synthesis of Camellia oleifera shell-derived hard carbon as an anode material for lithium-ion batteries

A comparatively facile and ecofriendly process has been developed to synthesize porous carbon materials from Camellia oleifera shells. Potassium carbonate solution (K₂CO₃) impregnation is introduced to modify the functional groups on the surface of Camellia oleifera shells, which may play a role in promoting the development of pore structure during carbonization treatment. Moreover, a small amount of naturally embedded nitrogen and sulfur in the Camellia oleifera shells can also bring about the formation of pores. The Camellia oleifera shell-derived carbon has a large specific surface area of 1479 m² g⁻¹ with a total pore volume of 0.832 cm³ g⁻¹ after being carbonized at 900 °C for 1 h. Furthermore, when used as an anode for lithium-ion batteries, the sample shows superior electrochemical performance with a specific capacity of 483 mA h g⁻¹ after 100 cycles measured at 200 mA g⁻¹ current density. Surprisingly, the specific capacity is even gradually increased with cycling. In addition, this sample exhibits almost 100% retention capacity after 250 cycles at a current density of 200 mA g⁻¹.

Bio-waste Camellia oleifera shells (COS) are converted into porous carbon by a two-step method.     

Facile synthesis of perovskite CeMnO3 nanofibers as an anode material for high performance lithium-ion batteries

A facile synthesis of perovskite-type CeMnO₃ nanofibers as a high performance anode material for lithium-ion batteries was demonstrated. The nanofibers were prepared by the electrospinning technique. The characterization of CeMnO₃ nanofibers was carried out by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy. SEM images manifested nanofibers with a diameter of 470 nm having a rough surface with a porous structure. TEM images were consistent with the observations from the SEM images. The electrochemical properties of CeMnO₃ perovskite in lithium-ion batteries were investigated. The CeMnO₃ anode exhibited a discharge capacity of 2159 mA h g⁻¹ with a coulombic efficiency of 93.79%. In addition, a high cycle stability and a capacity of 276 mA h g⁻¹ at the current density of 1000 mA g⁻¹ can be effectively maintained due to the high Li⁺ conductivity in the CeMnO₃ anode. This study could provide an efficient and potential application of perovskite-type CeMnO₃ nanofibers in lithium-ion batteries.

A facile synthesis of perovskite-type CeMnO₃ nanofibers as a high performance anode material for lithium-ion batteries was demonstrated.     

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 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.     

Facile synthesis of carbon and oxygen vacancy co-modified TiNb6O17 as an anode material for lithium-ion batteries

Titanium niobium oxides (TNOs), benefitting from their large specific capacity and Wadsley–Roth shear structure, are competitive anode materials for high-energy density and high-rate lithium-ion batteries. Herein, carbon and oxygen vacancy co-modified TiNb₆O₁₇ (A-TNO) was synthesized through a facile sol–gel reaction with subsequent heat treatment and ball-milling. Characterizations indicated that A-TNO is composed of nanosized primary particles, and the carbon content is about 0.7 wt%. The nanoparticles increase the contact area of the electrode and electrolyte and shorten the lithium-ion diffusion distance. The carbon and oxygen vacancies decrease the charge transfer resistance and enhance the Li-ion diffusion coefficient of the obtained anode material. As a result of these advantages, A-TNO exhibits excellent rate performance (208 and 177 mA h g⁻¹ at 10C and 20C, respectively). This work reveals that A-TNO possesses good electrochemical performance and has a facile preparation process, thus A-TNO is believed to be a potential anode material for large-scale applications.

Carbon and oxygen vacancy co-modified TiNb₆O₁₇ is synthesized through a facile sol–gel reaction. It possesses good electrochemical performance, thus is believed to be a potential anode material for large-scale applications.     

Controllable synthesis of nanostructured ZnCo2O4 as high-performance anode materials for lithium-ion batteries

Nanostructured ZnCo₂O₄ anode materials for lithium-ion batteries (LIBs) have been successfully prepared by a two-step process, combining facile and concise electrospinning and simple post-treatment techniques. Three different structured ZnCo₂O₄ anodes (nanoparticles, nanotubes and nanowires) can be prepared by simply adjusting the ratio of metallic salt and PVP in the precursor solutions. Charge–discharge tests and cyclic voltammetry (CV) have been conducted to evaluate the lithium storage performances of ZnCo₂O₄ anodes, particularly for ZnCo₂O₄ nanotubes obtained from a weight ratio 2 : 4 of metallic salt and PVP polymer in the precursor solution. Remarkably, ZnCo₂O₄ nanotubes exhibit high specific capacity, good rate property, and long cycling stability. Reversible capacity is still maintained at 1180.8 mA h g⁻¹ after 275 cycles at a current density of 200 mA g⁻¹. In case of rate capability, even after cycling at the 2000 mA g⁻¹ current density, the capacity could recover to 684 mA h g⁻¹. The brilliant electrochemical properties of the ZnCo₂O₄ anodes make them promising anodes for LIBs and other energy storage applications.

ZnCo₂O₄ nanoparticles, nanotubes, and nanofibers can be controllably prepared by simply tuning the weight ratios of metallic salts and PVP polymer in the precursor solution.     

Hierarchical non-woven fabric NiO/TiO2 film as an efficient anode material for lithium-ion batteries

Hierarchical non-woven fabric NiO/TiO₂ film is prepared using a facile two-step synthesis by electrospinning and subsequent hydrothermal reaction to yield TiO₂ film decorated with NiO nanosheets. As an anode material for Li-ion batteries, the NiO/TiO₂ composite exhibits discharge and charge capacities of 1251.3 and 1284.3 mA h g⁻¹, with good cycling performance and rate capability. Characterization shows that the NiO nanosheets are distributed on the surface of the TiO₂ nanofibers. The total specific capacity of NiO/TiO₂ is higher than the sum of pure TiO₂ and NiO, indicating a positive synergistic effect of TiO₂ and NiO on the improvement of electrochemical performance. The results suggest that non-woven fabric NiO/TiO₂ film is a promising anode material for lithium ion batteries.

Positive synergistic effect of TiO₂ and NiO of NiO/TiO₂ non-woven fabric electrode improves the electrochemical performance.     

First-principles calculations of an asymmetric MoO2/graphene nanocomposite as the anode material for lithium-ion batteries

Previous work on the synthesis and preparation of MoO₂/graphene nanocomposites (MoO₂/G) indicates that MoO₂/G is a good anode material for lithium-ion batteries (LIBS). In this work, we used larger super-cells than those used previously to theoretically construct an asymmetric MoO₂/G nanocomposite with smaller lattice mismatch. We then calculated the structural, electronic and Li atom diffusion properties of MoO₂/G using first-principles calculations based on density functional theory. The results show that asymmetric MoO₂/G has metallic properties, good stability and a low Li atom diffusion barrier because of the charge transfer induced by van der Waals interactions. The Li diffusion barriers in the interlayer of MoO₂/G are in the range of 0.02–0.29 eV, depending on the relative positions of the Li atom and the MoO₂ and the C atoms in the graphene layer. The Li diffusion barriers on the outside layers of the MoO₂/G nanocomposite are smaller than those of its pristine materials (MoO₂ and graphene). These results are consistent with experimental results. The adsorption of Li atoms in the interlayer of the nanocomposite further promotes the adsorption of Li atoms on the outside sites of the MoO₂ layer. Hence, the specific capacity of the MoO₂/G nanocomposite is larger than 1682 mA h g⁻¹. These properties all indicate that MoO₂/G is a good anode material for LIBS.

The structure of a nanocomposite constructed from MoO₂ and graphene and its Li atom adsorption and diffusion properties.     

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.     

Porous hydrogen substituted graphyne as a promising anode for lithium-ion batteries

Porous hydrogen substituted graphyne (HsGY) has been considered as a promising candidate for anode material due to its excellent electrochemical properties. In this work, we found that monolayer and bilayer HsGY are good electrodes for high charge capacity lithium-ion batteries based on density functional theory calculations. Mechanical tests reveal that monolayer and bilayer HsGY exhibit excellent mechanical properties, including large critical strains (>25%) and high in-plane stiffness (>200 N m⁻¹). The bilayer HsGY displays ultrahigh stiffness (400.27 N m⁻¹). Li adsorption on bilayer HsGY is stronger than that on the monolayer HsGY. Moreover, Li diffusion on the surfaces of monolayer and bilayer HsGY has low energy barriers (<0.5 eV). Our calculation results suggest that HsGY may contain the highest theoretical charge capacity among two-dimensional (2D) materials studied so far, with ultrahigh Li capacities of 3378 and 2895 mA h g⁻¹ for monolayer and bilayer HsGY, respectively. Given these advantages, including large critical strain, high mechanical stiffness, strong adsorption, low diffusive energy barrier, and high charge capacity, we conclude that both monolayer and bilayer HsGY could be promising anode materials for lithium-ion batteries.

Porous hydrogen substituted graphyne (HsGY) has been considered as a promising candidate for anode material due to its excellent electrochemical properties.     

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

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

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

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.     

A self-assembled silicon/phenolic resin-based carbon core–shell nanocomposite as an anode material for lithium-ion batteries

Silicon, with advantages such as high theoretical capacity and relatively low working potential, has been regarded as promising when it is used for lithium-ion battery anodes. However, its practical application is impeded by the intrinsic low electrical conductivity and the dramatic volume change during the lithiation/delithiation process, which leads to a rapid capacity fading of the electrode. In this regard, we design silicon nanoparticles homogeneously coated with a phenolic resin-based carbon layer as a core–shell nanocomposite via a facile self-assembly method followed by carbonization. The surrounding carbon shell, confirmed by transmission electron microscopy and Raman spectroscopy, is not only beneficial to the formation of a stable solid electrolyte interface film, but the electrical conductivity of the electrode is also enhanced. A high and stable specific capacity of nearly 1000 mA h g⁻¹ is achieved at C/3 after 200 cycles with a coulombic efficiency of >99.6%. The entire synthesis process is quite simple and easy to scale up, thus having great potential for commercial applications.

A self-assembled silicon/phenolic resin-based carbon core–shell nanocomposite is reported, which exhibits a high and stable reversible capacity and good rate capability.     

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.     

Silica from diatom frustules as anode material for Li-ion batteries

In spite of its insulating nature, SiO₂ may be utilized as active anode material for Li-ion batteries. Synthetic SiO₂ will typically require sophisticated synthesis and/or activation procedures in order to obtain a satisfactory performance. Here, we report on diatom frustules as active anode material without the need for extensive activation procedures. These are composed primarily of silica, exhibiting sophisticated porous structures. Various means of optimizing the performance were investigated. These included carbon coating, the addition of fluoroethylene carbonate (FEC) and vinylene carbonate (VC) to the carbonate-based electrolyte, as well as activation by an initial potentiostatic hold step. The highest capacity (723 mA h g⁻¹) was obtained with composite electrodes with pristine diatom frustules and conventional carbon black as additive, with the capacity still increasing after 50 cycles. The capacity was around 624 mA h g⁻¹ after subtraction of the contributions from the carbon black. Carbon coated diatom frustules showed a slightly lower but stable capacity after 50 cycles (600 mA h g⁻¹ after subtraction of contributions from the carbon coating and the carbon black). By the use of electrochemical characterization methods, as well as post-mortem studies, differences in reaction mechanisms could be identified and attributed to the operating and processing parameters.

Silica derived from algae was used as anode material in Li-ion batteries, giving a capacity of more than 700 mA h g⁻¹.     

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.     

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.     

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.     

Hierarchical NiO nanobelt film array as an anode for lithium-ion batteries with enhanced electrochemical performance

In this study, an ultrathin 2-dimensional hierarchical nickel oxide nanobelt film array was successfully assembled and grown on a Ni substrate as a binder-free electrode material for lithium ion batteries. In the typical synthesis process, the evolution of the nickel oxide array structure was controlled by adjusting the amount of surfactant, duration of reaction time and hydrothermal temperature. By virtue of the beneficial structural characteristics of the nanobelt film array, the as-obtained NiO array electrode exhibits excellent lithium storage capacity (1035 mA h g⁻¹ at 0.2C after 70 cycles and 839 mA h g⁻¹ at 0.5C after 70 cycles) for LIBs. This excellent electrochemical performance is attributed to the nanobelt film (3–5 nm thickness) array structures, which have immense open spaces that offer more Li⁺ storage active sites and adequate buffering space to reduce internal mechanical stress and shorten the Li⁺ diffusion distance. Additionally, this array structure is designed to achieve a binder-free and non-conductive additive electrode without complex coating and compressing during the electrode preparation process.

In this study, an ultrathin 2-dimensional hierarchical nickel oxide nanobelt film array was successfully assembled and grown on a Ni substrate as a binder-free electrode material for lithium ion batteries.     

Carbon-coated SnO2 riveted on a reduced graphene oxide composite (C@SnO2/RGO) as an anode material for lithium-ion batteries

The research on graphene-based anode materials for high-performance lithium-ion batteries (LIBs) has been prevalent in recent years. In the present work, carbon-coated SnO₂ riveted on a reduced graphene oxide sheet composite (C@SnO₂/RGO) was fabricated using GO solution, SnCl₄, and glucose via a hydrothermal method after heat treatment. When the composite was exploited as an anode material for LIBs, the electrodes were found to exhibit a stable reversible discharge capacity of 843 mA h g⁻¹ at 100 mA g⁻¹ after 100 cycles with 99.5% coulombic efficiency (CE), and a specific capacity of 485 mA h g⁻¹ at 1000 mA g⁻¹ after 200 cycles; these values were higher than those for a sample without glucose (SnO₂/RGO) and a pure SnO₂ sample. The favourable electrochemical performances of the C@SnO₂/RGO electrodes may be attributed to the special double-carbon structure of the composite, which can effectively suppress the volume expansion of SnO₂ nanoparticles and facilitate the transfer rates of Li⁺ and electrons during the charge/discharge process.

The combined action of GO and glucose makes the SnO₂ dispersed uniformly. The synergistic effect of the unique double-carbon structure can effectively improve the electrical conductivity of the SnO₂ and strengthen lithium storage capability.