Design and synthesis of one-dimensional Co₃O₄/Co₃V₂O₈ hybrid nanowires with improved Li-storage properties

Abstract

A new nanostructure of one-dimensional Co₃O₄/Co₃V₂O₈ hybrid nanowires directly grown on Ti substrates with improved electrochemical Li-storage properties are successfully prepared by a simple hydrothermal strategy. The nanocomposites consist of the primary Co₃O₄ nanowires acting as the “core” and secondary Co₃V₂O₈ nanocrystals as the “shell” layer, which form one-dimensional tentacle-like nanostructure. When used as potential anodes for lithium ion batteries, the Co₃O₄/Co₃V₂O₈ hybrid nanowires exhibited an enhanced capacity with high initial discharge capacity of 1677 mA h g⁻¹ at 200 mA g⁻¹ and retained at 1251 mA h g⁻¹ after 200 cycles. Even when the current reached 5000 mA g⁻¹ the electrode can still maintain an average discharge capacity of 807 mA h g⁻¹. The enhanced electrochemical performances are attributed to unique hybrid nanowire architecture and an improved synergistic effect of two electrochemically component, ranking the hybrid nanostructure as a promising electrode material for high-performance energy storage systems.

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1 Introduction

With the popularization of electric vehicles, hybrid electric vehicles and intelligent electronic devices, lithium-ion batteries (LIBs) have been considered as one of the most important technologies in energy storage.^1,2 The increasing demand for improving the energy density while maintaining their high power density and long cycle life has been the key driving force in developing advanced electrode materials for LIBs.^3,4 Compared with the conventional commercial graphite material, transition metal oxides have drawn considerable attention by their high reversible capacity via different pathways, such as converting mechanisms (Co₃O₄,⁵ NiO,⁶ Fe₃O₄ (ref. 7)) alloying/dealloying processes (SnO₂ (ref. 8)), or inserting/extracting mechanisms (TiO₂ (ref. 9)). In particular, Co₃O₄ has great potential due to its high theoretical specific capacity (890 mA h g⁻¹),¹⁰ good chemical/thermal stability, and low cost synthesis.¹¹ However, one of critical problems limiting its further practical applications is its poor stability caused by the low electrical conductivity and large volume changes associated with Li⁺ insertion/extraction processes.¹²

To overcome the above drawback, one of effective approaches is to design electrodes with different nanostructures like nanosheets,¹³ nanowires¹⁴ and nanoparticles,¹⁵ etc., as nanostructured electrodes can provide increased electro-active sites, large specific surface areas and more space to accommodate the volume change during the cycling process.^16,17 Among various nanostructures, one-dimensional (1D) structures, especially when aligned directly on current collectors, has great potential because of their high electrical contact and enhanced pathways to Li⁺ transport kinetics.¹⁸ Another strategy is to fabricate various binary metal oxides, which could overcome the poor stability drawback of single-phase oxides via a suitable combination of different metal oxides.¹⁹ For instance, nanophase ZnCo₂O₄ has been used as an anode and exhibited a stable capacity of 900 mA h g⁻¹ even after 60 cycles, in which both alloy formation and displacement reaction takes place to enhance capacity and stability.²⁰ Metal vanadates, such as Cu₃V₂O₈,²¹ FeVO₄,²² Zn₃V₂O₈,²³ have also previously been identified as good anode candidates for LIBs. Recently, an attractive concept of hybridize nanocomposites is emerging.^24–27 Yang et al. synthesized branched α-Fe₂O₃/SnO₂ nano-heterostructures,²⁸ while Xue et al. developed SnO₂/MoO₃ core–shell nanobelts,²⁹ and Mai et al. reported hierarchical MnMoO₄/CoMoO₄ heterostructured nanowires,³⁰ etc. These hybrid composites are capable of exhibiting enhanced electrochemical activities due to improved synergistic effect of each component. In this method, many Co₃O₄-based hybrid electrode materials like NiO–Co₃O₄ nanoplate composite,³¹ Li₄Ti₅O₁₂/Co₃O₄ composite,³² SnO₂@Co₃O₄ hollow nano-spheres,³³ Co₃O₄/CoFe₂O₄ nanocomposite³⁴ have been used as the anode materials for LIBs.

Co₃V₂O₈, as an important functional material, has a wide range of applications in catalysis,³⁵ magnetic applications,³⁶ and energy storage.³⁷ Very recently, Co₃V₂O₈ multilayered nanosheets³⁸ and Co₃V₂O₈·nH₂O hollow hexagonal prismatic pencils³⁹ showed excellent lithium storage properties, demonstrating as impressive anode material for LIBs. Thus, it is advisable to design a hybridize composites composed of Co₃O₄ and Co₃V₂O₈ with a unique nanostructure for high-performance LIBs anode. In addition, our previous study showed that three-dimensional Co₃O₄/Co₃V₂O₈ hybrid composites exhibited excellent electrochemical supercapacitive activity.⁴⁰ However, there is no report on their application for LIBs.

In this work, we reported a facile, two-step hydrothermal method for synthesis of Co₃O₄/Co₃V₂O₈ hybrid nanowires, which was designed to enhance the electrochemical properties of Co₃O₄ as an anode material for LIBs. The designed 1D/0D (Co₃O₄/Co₃V₂O₈) hybrid nanostructure has the following advantages. On the one hand, the Co₃O₄ nanowires directly grown on Ti substrates acts as the backbone to ensure great electrical contact and quick electron transport. On the other hand, the Co₃V₂O₈ nanocrystals are well wrapped on the Co₃O₄ nanowires surfaces, which can provide an increased portion of exposed surface for enhanced electrochemical capacity for Li⁺ storage and serve as structural spacers to maintain the structural integrity of the Co₃O₄ nanowires during battery cycling. Compared to the pristine Co₃O₄, the designed Co₃O₄/Co₃V₂O₈ hybrid nanowires exhibit enhanced capacity, improved cycling property, and outstanding rate capability through a synergistic interplay between the two active materials.

2 Experimental section

2.1 Synthesis of 1D Co₃O₄/Co₃V₂O₈ hybrid nanowires

Synthesis of Co₃O₄ nanowires. The Co₃O₄ nanowires grown on Ti substrates were prepared by a routine hydrothermal synthesis method. In a typical procedure, 4 mmol of Co(NO₃)₂·6H₂O, 20 mmol of CO(NH₂)₂ and 8 mmol NH₄F were dissolved into 80 mL deionized water. Then the obtained homogeneous solution were transferred into a 100 mL Teflon-lined autoclave with insertion of a piece of clean Ti foil, and maintained at 120 °C for 4 h, and then annealed at 450 °C in air for 3 h.

Synthesis of Co₃O₄/Co₃V₂O₈ hybrid nanowires. In a typical procedure, 1.6 mmol NH₄VO₃ was dissolved into 80 mL of deionized water at 80 °C. Under severe stirring, 2.4 mmol CoCl₂·6H₂O was added to the NH₄VO₃ solution. Then the obtained solution was transferred to a 100 mL Teflon-lined autoclave. Subsequently, the Ti foil supported with Co₃O₄ nanowires was immersed into the reaction solution at 120 °C for different reaction time. Afterward, the sample was rinsed with distilled water several times, and then annealed at 450 °C in air for 2 h.

2.2 Materials characterization

Crystallite structures were characterized by X-ray Diffraction (XRD) using a Rigaku D/MAX 2400 diffractometer (Japan) with Cu Kα radiation (λ = 0.15418 nm). The microstructure and morphology was determined by field-emission Scanning Electron Microscope (SEM, JEOL, JSM-6701F, Japan) and Transmission Electron Microscope (TEM, JEOL, JEM-2010, Japan) with Energy Dispersive Spectrometer (EDS).

2.3 Electrochemical measurements

The Ti foil supporting Co₃O₄/Co₃V₂O₈ hybrid nanowires and Co₃O₄ nanowires were directly used for battery assembly. The mass loading of active material was calculated based on the weight difference of Ti foil before and after the growth of active material. The electrochemical tests were performed using a coin-type half cell (CR-2032) with the Li-metal circular foil as the counter and reference electrodes, 1 M LiPF₆ in ethylene carbonate (EC) and dimethyl carbonate (DMC) (1 : 1, by volume) as the electrolyte, and the polypropylene (PP) micro porous film (Celgard 2300) as the separator. All coin cells were tested using a battery program-control test system (LAND CT2001A, Wuhan China). Cyclic voltammetry (CV; 0.01–3 V, 0.2 mV s⁻¹) and electrochemical impedance spectroscopy (EIS; 100 kHz to 0.01 Hz, 5 mV s⁻¹) were carried out by an electrochemical workstation (CHI660C, Shanghai China) at different current rates with a voltage window of 0.01–3 V.

3 Results and discussion

3.1 Synthesis and characterization

The formation process of the Co₃O₄/Co₃V₂O₈ hybrid nanowires was illustrated in Fig. 1. First, the Co₃O₄ precursors were formed directly on the substrate under the hydrothermal conditions (step i). The resulting precursors were annealed in air and transformed into Co₃O₄ nanowires (step ii). Then, these nanowires were employed as seeds to induce the growth of Co₃V₂O₈ nanocrystals on their surface (step iii), because of the lower interfacial nucleation energy on the surface of Co₃O₄ nanowires under the hydrothermal treatments.

Fig. 1 Schematic illustration of the formation process of 1D Co₃O₄/Co₃V₂O₈ hybrid nanowires.

The hybrid nanowires had a mixed XRD pattern of cubic Co₃O₄ (JCPDF card no. 65-3103) and cubic Co₃V₂O₈ (JCPDF card no. 16-0675) (Fig. 2a). The diffraction peaks at 19.1, 31.4, 36.9, 44.9, 59.6 and 65.5 degree were associated with (111), (220), (311), (400), (511) and (440) reflections of cubic Co₃O₄, and the diffraction peaks at 35.7, 43.6, 57.6, and 63.3 degree are correlated to (311), (400), (511) and (440) reflections of cubic Co₃V₂O₈, indicating successful synthesis of well-oriented Co₃O₄/Co₃V₂O₈ nanowires. No additional peaks were observed, suggesting the high purity of the composite. The composition of as-synthesized samples has also been characterized by EDS (Fig. 2b). The Co₃O₄ nanowires consisted of Co and O elements, whereas the entire composites consisted of Co, V, and O, which further confirmed that the composites were mainly made of Co₃O₄ and Co₃V₂O₈. The Co : V atomic ratio of hybrids electrodes was around 4.8 : 0.7, corresponding to 32.1% (mass percentage) of Co₃V₂O₈ in nanocomposites. The presence of Cu peaks at the EDS spectrum originated from the Cu wafer used as a support of the samples of TEM observation.

Fig. 2 (a) XRD pattern of Co₃O₄/Co₃V₂O₈ hybrid nanowires. (b) EDS spectrums of Co₃O₄ nanowires and Co₃O₄/Co₃V₂O₈ hybrid nanowires.

The morphological and structural features of the as-prepared composites were characterized by SEM and TEM. As shown in Fig. 3a–d, the Co₃O₄ nanowires have an average diameters of around 50–100 nm, lengths of around several microns, and the as-prepared Co₃O₄/Co₃V₂O₈ hybrid nanowire exhibited 1D tentacle-like nanostructure with secondary Co₃V₂O₈ nanocrystals grown on the surface of primary Co₃O₄ nanowires. The TEM images of hybrid nanowires clearly revealed that the Co₃V₂O₈ nanocrystals were attached closely together to the surface of Co₃O₄ nanowires in Fig. 3e. These nanowires and nanocrystals were interconnected, and the heterojunctions between them could be clearly observed as shown by white circles in Fig. 3f, the boundaries between two Co₃V₂O₈ nanocrystals could be detected as shown by white dotted lines. Furthermore, the lattice spacings of the inner nanowire were 0.46 nm, correlated to the (311) plane of cubic Co₃O₄, and the lattice spacings of the outer nanocrystals are 0.21 and 0.24 nm, in good accord with the (400) and (311) plane of cubic Co₃V₂O₈, respectively, which agreed well with the results of XRD analysis. Fig. S1† was the SAED pattern of Co₃O₄/Co₃V₂O₈ hybrid nanowires, which demonstrates the polycrystalline nature of the hybrid nanowires.

Fig. 3 SEM images of (a) Co₃O₄ nanowires and (b, c) Co₃O₄/Co₃V₂O₈ hybrid nanowires. TEM images of (d) Co₃O₄ nanowires and (e) Co₃O₄/Co₃V₂O₈ hybrid nanowires. (f) High resolution TEM image of Co₃O₄/Co₃V₂O₈ hybrid nanowires.

To investigate the evolution process of Co₃O₄/Co₃V₂O₈ hybrid nanowires, a series of time-dependent experiments were conducted and the SEM images of the products with varied reactions times (step iii) were shown in Fig. S2.† The density and size of secondary Co₃V₂O₈ nanocrystals can be tuned by changing the reactant time in step iii. There was a few of Co₃V₂O₈ nanoparticles nucleated on the surface of the Co₃O₄ nanowires when the reactant time was 0.5 h (Fig. S2a†). These Co₃V₂O₈ nanoparticles elongated and formed nanocrystals with further increasing the reactant time to 1–2 h (Fig. S2b and c†). When the reactant time was further enlarged to 4 h, uniform hybrid nanowires were formed (Fig. 3c). After that, the overall morphology showed no change with the increase of reactant time to 6 h (Fig. S2d†).

3.2 Electrochemical evaluation of the electrodes

Cyclic voltammetry measurements were measured to evaluate the electrochemical reaction of the Co₃O₄/Co₃V₂O₈ hybrid nanowires at a scan rate of 0.2 mV s⁻¹, as shown in the Fig. 4a. During the first cycle, an irreversible cathodic peak at 0.66 V could be assigned to the reductions of Co₃O₄ to Co and the formation of solid electrolyte interphase (SEI) layer by the electrolyte decomposition. Another peak at around 0.33 V corresponded to the reductions of Co₃V₂O₈ into CoO and the reduction of CoO to Co, accompanied with the formation of Li_xV₂O₅.^38,39,41 The anodic scanning showed one broad peaks at 1.07–1.6 V derived from the extraction of Li⁺ from the Li_xV₂O₅ matrices, the decomposition of Li₂O and the phase transformation from Co to CoO. The peak at 2.06 V might be attributed to the further extraction of Li⁺ from the Li_x−yV₂O₅ matrices and the electrochemical oxidation of Co to Co₃O₄. After the first cycle, note that the CV peaks overlap well during the subsequent cycles, suggesting a high reversibility of the electrochemical reaction and a good cycling performance. The CV profiles of Co₃O₄ nanowires were also provided in Fig. S3a† for comparison.

Fig. 4 (a) CV curves of Co₃O₄/Co₃V₂O₈ hybrid nanowires for the first three cycles; (b) the charge–discharge profiles of Co₃O₄/Co₃V₂O₈ hybrid nanowires. (c) Comparison of cycling performance of Co₃O₄/Co₃V₂O₈ hybrid nanowires and Co₃O₄ nanowires at a current of 200 mA g⁻¹. (d) Rate capability testing at various current densities.

Fig. 4b showed the first three discharge–charge profiles of hybrid nanowires. During the initial discharge process, there were two apparent voltage plateaus appearing at 0.92 V and 0.65 V, in consistence with the above CV detection results. The Co₃O₄/Co₃V₂O₈ hybrid nanowires provided a high discharge capacity of 1677 mA h g⁻¹ at the first cycle. The discharge capacity of hybrid nanowires was much higher than their theoretical capacity owing to the formation of solid electrolyte interface (SEI) film, which was common for metal–oxide based electrodes.⁴² The discharge capacity of the hybrid nanowires mainly dropped at the subsequent cycles, and was retained at 1299 mA h g⁻¹ after 3 cycles, corresponding to an irreversible capacity loss due to the formation of SEI films and some undecomposed Li₂O.⁴³ For comparison, the initial discharge capacity of the Co₃O₄ nanowires was 1243 mA h g⁻¹ and retained at 836 mA h g⁻¹ after 3 cycles (Fig. S3b†), indicating that the hybrid nanowires lead to better reversibility of the electrochemical reaction during repeated charging–discharging processes.

Fig. 4c showed the comparative cycling performance of these two samples up to 200 cycles at a current density of 200 mA g⁻¹. The discharge capacity of Co₃O₄/Co₃V₂O₈ hybrid nanowires decreased in the initial 50 cycles, then maintained its capacity from the 50th to 100th cycle, but increased its capacity slowly after 100 cycles, and reached at a capacity of 1251 mA h g⁻¹ at the 200th cycle. Similar phenomena for the initial drop followed by gradual rise of the capacity has also been observed from various transition-metal oxide composite anode materials.^44,45 The reversible formation of a polymeric gel-like film came from the decomposition of electrolyte may be responsible for the capacity rise phenomena.⁴⁶ The discharge capacity of Co₃O₄ nanowires rapidly dropped to 531 mA h g⁻¹ after 200 cycles, which was much lower than that of hybrid nanowires. The capacity retention of the hybrid nanowires was also higher than the previous reports related on the Co₃O₄-based or Co₃V₂O₈-based electrode materials as summarized in Table S1.†

In addition to the improved cycling performance, the Co₃O₄/Co₃V₂O₈ nanowires also displayed significantly enhanced rate performance at various current rates (Fig. 4d). After every ten cycles at a current rate, the hybrid nanowires delivered an outstanding discharge capacity of 1276.4, 1150.5, 1044.6, and 949.1 mA h g⁻¹, when the current density increased stepwise to 100, 500, 1000, and 2000 mA g⁻¹, which is much higher than that of the Co₃O₄ electrode. Even when the current reached 5000 mA g⁻¹ the electrode can still maintain an average discharge capacity of 807 mA h g⁻¹. Upon decreasing the current density to 100 mA g⁻¹, an average discharge capacity of 1341 mA h g⁻¹, which was almost identical to the capacity of the second discharge cycle. Such significantly enhanced rate performance of the Co₃O₄/Co₃V₂O₈ nanowires in comparison with bare Co₃O₄ nanowires might be closely associated with their excellent conductivity.

Fig. 5 showed the Nyquist plots of the electrodes from 100 kHz to 0.01 Hz after 5 cycles at 200 mA g⁻¹ to prove the good performance of hybrid nanowire. Each plot consisted of a depressed semicircle in the high-frequency region and an inclined line in the low-frequency region. The semicircle was attributed to the sum total of the electrolyte resistance, the SEI resistance, and the charge transfer resistance, and the slope line might be related with mass transfer of lithium ions induced by lithium diffusion in the electrodes.⁴⁷ Obviously, the size of the semi-circle for the Co₃O₄/Co₃V₂O₈ hybrid nanowires was much smaller than Co₃O₄ nanowires, suggesting higher electrical conductivity and faster Li⁺ diffusion rates.

Fig. 5 The impedance spectra of Co₃O₄ nanowires and Co₃O₄/Co₃V₂O₈ hybrid nanowires after 5 cycles at 200 mA g⁻¹.

To further understand the enhanced electrochemical performance of 1D Co₃O₄/Co₃V₂O₈ hybrid nanowires, the morphological and structural characteristics of obtained samples after the cycling test at various current rates were further checked by SEM and TEM as shown in Fig. 6. As can be seen, Co₃O₄/Co₃V₂O₈ hybrid nanowires had no obvious change during the cycling process, but the Co₃O₄ nanowires were seriously crumbled and aggregated, which can account for the large difference in the rate capability of Co₃O₄/Co₃V₂O₈ and Co₃O₄ electrodes.

Fig. 6 SEM and TEM images of (a) the Co₃O₄ nanowires and (b) Co₃O₄/Co₃V₂O₈ hybrid nanowires electrode after the cycling test at various current rates.

As the experimental results discussed above, the remarkable performance of the Co₃O₄/Co₃V₂O₈ hybrid nanowires mainly originated from their unique nanostructure. Firstly, the Co₃O₄ nanowires directly grown on Ti substrates could serve as the backbone of the nanostructure for fast Li⁺ transport. Secondly, the secondary Co₃V₂O₈ nanocrystals grown on the surface of nanowires gave an increased portion of exposed surface to offer more active sites for Li⁺ ions access, and served as structural spacers to reduce the aggregation of inner nanowires, resulting in enhanced capacity and cycling stability. Furthermore, the excellent electrochemical performance could be attributed to the synergistic effects of the two metal oxides in lithiation–delithiation process and its better electronic conductivity.

4 Conclusion

We had demonstrated a stepwise and cost-effective hydrothermal approach to fabricate the novel Co₃O₄/Co₃V₂O₈ hybrid nanowires, which was composed of 1D Co₃O₄ nanowires decorated with 0D Co₃V₂O₈ nanocrystals for LIBs application. The Co₃O₄/Co₃V₂O₈ hybrid nanowires exhibited an enhanced capacity with high initial discharge capacity of 1677 mA h g⁻¹ at 200 mA g⁻¹ and retained at 1251 mA h g⁻¹ after 200 cycles. Even when the current reached 5000 mA g⁻¹, the electrode can still maintain an average discharge capacity of 807 mA h g⁻¹, indicating that the hybrid nanowires possessed enhanced Li-storage performance compared with pure Co₃O₄ nanowires. The enhanced electrochemical performance was attributed to unique nanostructure and the synergetic effects of different components. Our work also demonstrates the importance and great potential for Co₃O₄/Co₃V₂O₈ nanocomposite in the development of high performance energy storage electrode. Such nanocomposite might also be used in broad fields including supercapacitors, fuel cells, and water splitting.

Acknowledgements

The work was supported by the National Natural Science Foundation of China (no. 51362018 and no. 21163010).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra02502a

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