Sacrificial template formation of CoMoO₄ hollow nanostructures constructed by ultrathin nanosheets for robust lithium storage

Abstract

Hierarchical hollow structures composed of two-dimensional nanostructures are desirable and challenging research subjects in the synthesis and application of nanomaterials. In the present work, CoMoO₄ hollow nanostructures assembled by nanosheet subunits were successfully prepared via a facile solvothermal method using SiO₂ nanospheres as the sacrificial template. In the synthesis process, the formation and assembly of CoMoO₄ nanosheets, as well as the template removal, were carried out simultaneously, thus avoiding the cumbersome multi-step process of the traditional template method. The obtained nanosheets-constructed hollow nanostructures possess combined structural advantages for robust electrochemical properties as anode materials for lithium-ion batteries, such as a porous and robust framework, high surface area for electrode/electrolyte contact, reduced lithium diffusion path, and a hollow interior to accommodate the volume variation associated with the electrochemical cycles. Hence, the CoMoO₄ electrode was able to deliver good lithium storage properties, including high specific capacity (1066 mA h g⁻¹ at a current density of 500 mA g⁻¹ after 200 cycles), good cyclic stability, and an especially excellent rate performance.

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Introduction

Two-dimensional (2D) nanomaterials, including graphene, metal sulfides, metal oxides, and metal hydroxides, have aroused intense research interest due to their intriguing physical, chemical, and electric advantages and fascinating applications in catalysis, sensing, capacitors, and lithium-ion batteries (LIBs).^1–8 Specifically, 2D-nanostructured metal oxides and metal sulfides, as promising high capacity anode materials for lithium-ion batteries storing lithium via a conversion reaction, have been demonstrated to manifest high-performance lithium storage properties. For instance, our previously reported SnO₂ nanosheets delivered an ultralong lifespan of over 1000 cycles as well as high capacity owing to its specific 2D features.⁸ Recently, hollow structures constructed by 2D nanosheets have attracted special focus in LIB applications because their hierarchical architectures possess additional structural advantages for energy storage;^4,9–11 however, their synthesis is still challenging.

Very recently, the ternary transitional oxide of CoMoO₄ with a high theoretical capacity (980 mA h g⁻¹) was demonstrated as a high-performance LIB anode material.^12–16 Over the last decade, various CoMoO₄ nanostructures, including particles,^12,19 nanorods,¹³ nanosheets,^14,17,18 microspheres,¹⁵ and hollow nanostructures,¹⁷ have been synthesized. For instance, Baskar et al. prepared CoMoO₄ in a large scale using a facile method.^17,19 However, the synthesis and lithium storage application of CoMoO₄ 2D-nanosheets-constructed 3D hollow nanostructures have rarely been reported. Therefore, in this work, we present a sacrificial template method to prepare CoMoO₄ hollow nanostructures assembled by ultrathin nanosheets as high-performance LIB anode materials for the first time. The presented synthesis strategy simultaneously achieves the formation and assembly of nanosheets as well as the template removal, thus avoiding the cumbersome multi-step process of the traditional template method. The 3D architectures synergistically combine the advantages of hollow interior space and ultrathin 2D features, such as faster lithium diffusion, improved contact between the active materials and electrolyte, and a buffered volume variation, thus paving the way to realize its potential for lithium storage. Therefore, the CoMoO₄ nanosheets-constructed hollow nanostructures exhibit high reversible capacity and excellent rate performance.

Experimental

The synthesis of CoMoO₄ nanosheets-constructed hollow nanostructures involves two steps: preparation of the SiO₂ nanosphere template and the subsequent hydrothermal process. First, the SiO₂ nanospheres were synthesized via a Stöber method. Cetyl trimethyl ammonium bromide (1.2 g) was dissolved in a mixed solution of 40 mL of ethanol and 60 mL of deionized water. After 0.8 mL of ammonium hydroxide (28%) was added, 1.6 mL of tetraethyl orthosilicate was added dropwise into the mixed solution. After stirring for 10 h, the white precipitate was collected by centrifugation, washed with ethanol and water in turn three times, and then vacuum dried at 60 °C.

Using the as-obtained SiO₂ nanospheres as sacrificial templates, a hydrothermal method was developed for the synthesis of CoMoO₄ nanosheets-assembled hollow nanostructures. Importantly, NH₄F was used to etch the SiO₂ spheres accompanied by the formation of CoMoO₄ hollow nanostructures during the hydrothermal treatment. Typically, 200 mg of SiO₂ nanospheres and 0.5 g of polyvinyl pyrrolidone (PVP, K30) were first dispersed in 120 mL water. After ultrasonication for 1 h, 0.41 g of CoNO₃·6H₂O and 0.34 g of Na₂MoO₄·2H₂O were added under stirring. Then, 0.088 g of NH₄F was added and the solution was stirred for 30 min to obtain a pink suspension, which was transferred into a Teflon-lined stainless steel autoclave and kept at 180 °C for 48 h in an oven. After cooling down to room temperature naturally, the pink precipitate was collected, washed with ethanol and water in turn, and dried at 60 °C to obtain the final CoMoO₄ nanosheets-constructed hollow nanostructures. Herein, the CoMoO₄ particles were also prepared without a SiO₂ template.

Scanning electron microscopy (SEM, JEOL JSM-7500F), transmission electron microscopy (TEM, JEOL, JEM-2100), and powder X-ray diffraction (XRD, Philips X'-pert X-ray diffractometer) were used to measure the samples. N₂ adsorption/desorption isotherms of the CoMoO₄ nanosheets-constructed hollow nanostructures were obtained using the Micromeritics TriStar (Micromeritics Co. Ltd., UK) at 77 K. The total specific surface area was determined using the multipoint Brunauer–Emmett–Teller (BET) method.

Electrochemical lithium storage was achieved with CR2016-type coin cells. The active materials (CoMoO₄ nanosheets-constructed hollow nanostructures or CoMoO₄ particles), acetylene black, and polyvinylidene fluoride (PVDF) (8 : 1 : 1 in weight) were mixed together. The mixture was compressed onto a copper foil with a diameter of 2 mm (1.5–2 mg cm⁻²) and then dried at 120 °C for 24 h in a vacuum oven to fabricate the working electrode. A metallic lithium sheet with a diameter of 15 mm was used as the negative electrode. The electrolyte was 1 M LiPF₆ in ethylene carbonate (EC)/dimethyl carbonate (DMC)/ethyl methyl carbonate (EMC) (1 : 1 : 1 in volume), and ∼0.15 mL of the electrolyte was used per cell. The Clegard 2300 microporous film was used as the separator. The cells were assembled in a high-purity argon-filled glove box. Charge–discharge tests were conducted on a LAND CT2001A Battery Cycler ranging from 0.01 to 3 V. Cyclic voltammetry (CV) tests were obtained on a CHI760D instrument (CH Instruments, Shanghai, China) using the coin cell at a scan rate of 0.05 mV s⁻¹. Electrochemical impedance spectroscopy (EIS) measurements were also performed at a frequency of between 0.1 Hz and 100 kHz.

Results and discussion

Fig. 1a and b show the SEM images of the SiO₂ template, in which highly uniform spheres with a narrow size distribution (500–700 nm in diameter) can be observed. The SEM images of the CoMoO₄ nanosheets-constructed hollow nanostructures are shown in Fig. 2a and b. One can see that CoMoO₄ micro-flowers are constructed by ultrathin 2D nanosheets. The nanosheets show good flexibility and are several hundreds of nanometers in size. As shown in Fig. 2d, the CoMoO₄ material prepared without the SiO₂ template comprises irregular particles, suggesting that the SiO₂ template plays a key role in obtaining CoMoO₄ nanosheets-constructed hollow nanostructures. The XRD pattern of the CoMoO₄ nanosheets-constructed hollow nanostructures is presented to reveal the crystallinity. Clearly, all the diffraction peaks labelled in Fig. 2c can be well indexed to the monoclinic phase CoMoO₄ with a C2/m space group (JCPDS 21-0868), and the calculated lattice parameters of a = 10.21 Å, b = 9.27 Å, and c = 7.02 Å are in good agreement with the theoretical values. The good crystallization of CoMoO₄ is confirmed by the sharp and strong peaks.

Fig. 1 (a and b) SEM images of SiO₂ nanospheres.

Fig. 2 (a and b) SEM images and (c) XRD pattern of the CoMoO₄ nanosheets-constructed hollow nanostructures. (d) SEM images of the CoMoO₄ particles.

The TEM images were also obtained to reveal the structures of the CoMoO₄ nanosheets-constructed hollow nanostructures. As shown in Fig. 3a, nanostructures with a hollow interior can be clearly observed, and are constructed by numerous radially standing ultrathin 2D nanosheets. As labelled in Fig. 3a, the hollow interior of the nanostructures shows an almost ideal circular shape, which is highly in agreement with the SiO₂ sacrificial template in size. So it can be proposed that, the CoMoO₄ nanosheets are formed and assembled on the surface of the SiO₂ template, and the SiO₂ is simultaneously etched by NH₄F during the present hydrothermal synthesis strategy, thus in situ inducing the template-derived hollow interior. A magnified HRTEM image of the nanosheet subunits (Fig. 3b) clearly shows that the nanosheet possesses a small thickness of 6 nm. Also, the dominant lattice with a d spacing of 0.34 nm can be detected clearly, which corresponds to the (002) crystal plane of CoMoO₄. The selected electron diffraction (SAED) pattern (Fig. 3c) also confirms the polycrystalline feature of the CoMoO₄ hollow nanostructures. The diffraction rings are well assigned to the monoclinic CoMoO₄ phase, which is consistent with the XRD result. In addition, N₂ sorption measurements of the CoMoO₄ nanosheets-constructed hollow nanostructures were conducted and shown in Fig. 3d, and in which CoMoO₄ shows a high BET surface area of 78.2 m² g⁻¹. This value is higher than the reported BET surface area for other CoMoO₄ materials,^12,15 but is comparable with the results for similar hollow nanostructures,^9,10 and is smaller than the result for the CoMoO₄ nanosheets.¹⁸ In comparison with this, the CoMoO₄ particles (Fig. S1†) only show a low surface area of 27.5 m² g⁻¹. Such hierarchical hollow nanostructures provide a high surface area for the active materials and electrolyte contact, and tunnels for the rapid transfer of electrolytes and lithium, as well as additionally providing an interior to accommodate the volume variation, thus remarkably improving the electrochemical lithium storage properties of CoMoO₄ anode materials for LIBs.

Fig. 3 (a) TEM image, (b) HRTEM image, and (c) the corresponding SAED pattern of the CoMoO₄ nanosheets-constructed hollow nanostructures. (d) The N₂ adsorption/desorption isotherms of the CoMoO₄ nanosheets-constructed hollow nanostructures. The inset shows the corresponding pore size distribution.

The electrochemical lithium storage properties of these CoMoO₄ nanosheets-constructed hollow nanostructures were investigated. As expected, the CoMoO₄ nanosheets-constructed hollow nanostructures exhibited excellent electrochemical performance. Fig. 4a shows the first, second, and 100th charge/discharge voltage profiles of a CoMoO₄ electrode at a constant current density of 500 mA g⁻¹ in the voltage range between 0.01 and 3 V. During the initial discharge curve, the voltage plateau at 0.8–0.6 V corresponds to the reduction of Mo⁶⁺ to Mo⁴⁺.^12–15 Another distinct voltage at around 0.6–0.2 V is assigned to the reduction of Mo⁴⁺ to metallic Mo and Co²⁺ to metallic Co. Both of these plateaus shift slightly upward in the subsequent cycles, which is consistent with the previous reports.^12–15 In comparison with this, CoMoO₄ particles (Fig. S2†) show similar voltage profiles but poor capacity retention. The curves at the second and 100th cycle overlap very well, suggesting the good cyclic stability of the CoMoO₄ hollow nanostructures. The charge–discharge process of the hollow-structured CoMoO₄ electrode was also revealed by the CV technique, which was found to be consistent with the galvanostatic test results. In Fig. 4b, three cathodic peaks of 1.4, 0.48, and 0.15 V can be observed at the first cycle. The initial two peaks are normally assigned to the reduction of Mo⁶⁺ to Mo and Co²⁺ to Co, which also show positive shifts during the following cycles.^12,14 The peak at 0.15 V is usually attributed to the formation of a solid electrolyte interphase (SEI) layer, which is irreversible and cannot be observed during the subsequent cycles. During the anodic scans, two peaks at about 1.5 and 1.8 V are seen and are associated with the oxidation of Co to CoO and Mo to MoO₃. Also, the good overlap of the 2nd cycle and 5th cycle further reveals the good cyclic stability of the CoMoO₄ hollow nanostructures.

Fig. 4 Electrochemical lithium storage properties of CoMoO₄ nanosheets-constructed hollow nanostructures: (a) galvanostatic charge–discharge profiles at the first, second, and 100th cycles at a current density of 500 mA g⁻¹; (b) CV curve; (c) cyclic performance at 500 mA g⁻¹; (d) rate capability at current densities ranging from 0.5 to 10 A g⁻¹. (e) EIS spectra of CoMoO₄ hollow structures and particles before and after 100 cycles.

Besides, the cyclic performance at 500 mA g⁻¹ (Fig. 4c) further indicates the stable cyclic performance of over 200 electrochemical cycles. The CoMoO₄ hollow nanostructures exhibit high initial discharge and charge capacities of 1596 and 1151 mA h g⁻¹, respectively, with a corresponding coulombic efficiency of 72%. The low initial coulombic efficiency is due to the decomposition of the electrolyte and the formation of an SEI layer during the first charge–discharge process, which consumes extra Li⁺. The coulombic efficiency increases remarkably to ∼97% in the second cycle and then keeps constant at over ∼98% all through the 200 cycles, revealing the high reversibility of the CoMoO₄ nanosheets-constructed hollow nanostructures electrode. Only a tiny capacity fade is detected from the second cycle onward, and a high discharge capacity of 1066 mA h g⁻¹ is retained even after 200 cycles, thus delivering a high capacity retention of 93% compared with the value at the second cycle. Interestingly, this value is higher than the theoretical capacity of 980 mA h g⁻¹ for CoMoO₄. This is commonly observed for nanostructured metal oxide anode materials and is usually assigned to the 2D specific structures of the CoMoO₄ subunit nanosheets, which supply many interfacial sites for additional lithium insertion.^20–23 In contrast with this, the as-prepared CoMoO₄ particles show very poor cyclic stability, and the capacity decreases dramatically from the initial 857 to 236 mA h g⁻¹ at the 100th cycles. Noting this, the excellent stable cyclic performance of our present CoMoO₄ nanosheets-constructed hollow nanostructures are among the best results ever reported. The reversible capacity of the CoMoO₄ electrode obtained at a high current density of 500 mA g⁻¹ is even higher than most of the reported CoMoO₄-based electrodes at relatively lower current densities.^12–14 Moreover, CoMoO₄ hollow nanostructures exhibit higher reversible capacity than the recently reported Co₃O₄ nanoplates² and SnO₂ nanosheets,⁸ and also show improved longevity compared to double-shelled SnO₂@C hollow spheres²⁴ and Si–graphene composite.²⁵

Impressively, the CoMoO₄ nanosheets-constructed hollow nanostructures deliver the best rate performance ever reported for CoMoO₄-based anodes, as shown in Fig. 4d, which is a characteristic highly desired for achieving high-performance LIBs. The CoMoO₄ nanosheets-constructed hollow nanostructures manifest high discharge capacities of 989, 871, 742, and 610 mA h g⁻¹ at current densities of 1, 2, 4, and 8 A g⁻¹, respectively. These results are better than the reported state of art results, including that of 784 mA h g⁻¹ at 2 A g⁻¹ for CoMoO₄ nanorods–graphene composites,¹³ 611 mA h g⁻¹ at 1.5 A g⁻¹ obtained from the 3D CoMoO₄ electrode,¹⁴ and that of 783 mA h g⁻¹ at 1.5 A g⁻¹ for rattle-type CoMoO₄ microspheres.¹⁵ Even at a very high current density of 10 A g⁻¹, a high reversible capacity of 470 mA h g⁻¹ could still be achieved, which is much higher than the theoretical capacity of 372 mA h g⁻¹ for graphite. The EIS measurements of the two electrodes are presented in Fig. 4e. Clearly, all the EIS spectra show a quasi-semicircle in the high frequency region and an inclined linear part at low frequency. Based on the semicircle, CoMoO₄ hollow structures show a charge transfer resistance (R_ct) of 83.4 and 140.5 Ω before and after the cycle, respectively, which are much smaller vales than the corresponding values of 118.7 and 226.5 Ω for CoMoO₄ particles, thus showing their lower charge transfer resistance and better electrochemical dynamic behavior. Importantly, the CoMoO₄ hollow structures deliver much smaller resistance changes (ΔR_ct = 57.1 Ω) before and after the cycle in comparison with that of CoMoO₄ particles (107.8 Ω), suggesting an improved cyclic stability.

To explore the root of the stable cyclic performance, the TEM image of the CoMoO₄ electrode after 100 cycles at 500 mA g⁻¹ was obtained and is presented in Fig. 5a; also, the corresponding SAED pattern is presented in Fig. 5b. In Fig. 5a, the hollow structures of the CoMoO₄ aggregates collapse after such a long cycle test. However, the CoMoO₄ subunits can still maintain their sheet-like structures, indicating the excellent mechanical stability of the CoMoO₄ nanosheets. Therefore, the stable 2D structures play a key role in the good cycling durability of the CoMoO₄ electrode. The corresponding SAED pattern reveals the amorphous phase of CoMoO₄ after the 100th cycle test.

Fig. 5 (a) TEM image and (b) corresponding SAED pattern of the CoMoO₄ nanosheets-constructed hollow nanostructures after 100 electrochemical cycles at 500 mA g⁻¹.

The excellent electrochemical lithium storage properties of the CoMoO₄ nanosheets-constructed hollow nanostructures, particularly their excellent high-rate performance, can be assigned to their specific nanostructure. The nanosheet-subunits-constructed hierarchical architecture and the hollow feature provide a porous and robust framework, a high surface area for electrode/electrolyte contact, and a reduced diffusion path for lithium and the electrolyte. All of the above advantages could remarkably boost the electrochemical reaction kinetics. Moreover, such an architecture will better buffer the huge volume changes during the repeated lithiation/delithiation. Therefore, a high reversible capacity, stable cyclic performance, and good rate capability can be obtained from these CoMoO₄ nanosheets-constructed hollow nanostructures.

Conclusions

In summary, a sacrificial template-assistant solvothermal method has been developed to construct CoMoO₄ hierarchical hollow nanostructures assembled by nanosheet subunits. SiO₂ was used as template for the support of as-formed nanosheet subunits, and was simultaneously etched and in situ give rise to the formation of a hollow interior. In virtue of the structural benefits, the CoMoO₄ hierarchical hollow nanostructures exhibited robust electrochemical lithium storage properties such as high reversible capacity, stable cyclic performance, and superior rate capability, thus indicating their promise as anode materials for LIBs. In addition, we believe that the developed sacrificial method could be employed for the fabrication of other metal oxides/sulfides hollow structures for potential applications in energy conversion and storage.

Acknowledgements

We gratefully acknowledge for Shandong Provincial Natural Science Foundation, China (ZR2014JL015, ZR2014EMM004).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra08534b

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