Facile fabrication of highly porous Co₃O₄ nanobelts as anode materials for lithium-ion batteries

Abstract

Highly porous Co₃O₄ nanobelts were successfully synthesized by using a hydrothermal technique, followed by calcination of the Co(OH)₂ precursor. The as-prepared Co₃O₄ nanobelts were analyzed by scanning electron microscopy, X-ray power diffraction, transmission electron microscopy, and Brunauer–Emmett–Teller methods. The electrochemical properties of porous Co₃O₄ nanobelts were examined by cyclic voltammetry and galvanostatic charge–discharge studies. Owning to the unique 2D structural features, the Co₃O₄ nanobelts exhibited a high specific capacity of 857 after 60 cycles at a current density of 100 mA g⁻¹, and good cycling stability. These exceptional electrochemical performances could be attributed to the remarkable structural feature with a high surface area and large amounts of nanopores within the surface of nanobelts, which can provide large contact areas between electrolyte and active materials for electrolyte diffusion, improve structural stability and buffer volume expansion during the Li⁺ insertion/extraction processes.

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1. Introduction

Over the past few years, rechargeable lithium-ion batteries (LIBs) have become the power source of choice for modern portable electronic equipment and have been used wildly in energy storage and conversion devices owning to their high energy density, light weight and long cycle life.^1–4 Recently, transition metal oxides (TMOs) are intensively investigated as promising anode materials (e.g., Co₃O₄,⁵ NiO,⁶ Mn₂O₃,⁷ MnO,⁸ Fe₂O₃,⁹ and Mn_1.2Fe_1.8O₄ (ref. 10)) because of their higher theoretical capacities than that of commercial graphite (372 mA h g⁻¹). Among these available TMOs anode materials, Co₃O₄ has attracted much attention as a promising candidates to replace currently commercial graphite anode for next generation LIBs, because Co₃O₄ exhibits a high theoretical capacity of 890 mA h g⁻¹, which is more than twice that of graphite (372 mA h g⁻¹).^11,12 However, like other TMOs, the application of Co₃O₄ as a practical electrode material is still hindered by large volume expansion and subsequently electrode pulverization during the charge/discharge processes, which may result in a large reversible capacity fading and poor cycling performance.^13–15 As a consequence, considerate efforts have recently been focused on searching for Co₃O₄-based electrode materials with unique structures to solve the inherent poor electrical conductivity and large volume expansion upon cycling. The hybrid structures of Co₃O₄ with carbon materials can be considered as an effective route to solve these intrinsic drawbacks, because the carbon materials in the structures can largely improve the electrical conductivity of the electrode and cushion the volume expansion during the charge/discharge processes.^16,17 However, the introduction of carbon materials would reduce the specific capacity of Co₃O₄ due to the low theoretical capacity of carbon. In addition, the fabrication of well designed Co₃O₄/carbon composites is complex and remains a challenge. The recent studies show that the electrochemical performance of Co₃O₄ also largely depends on the morphology of its nanostructures.^18,19 For example, Yan et al. reported that Co₃O₄ nanoparticles exhibited excellent Li-storage performance as anode materials for LIBs.²⁰ Therefore, it is desirable to design and fabricate unique nanostructures to realize high electrochemical performance of Co₃O₄ as anode materials for LIBs.

Recently, various Co₃O₄ nanostructures, including nanocubes,²¹ nanoflowers,²² nanotubes,²³ and microspheres,²⁴ have been explored as anode materials for LIBs, and indeed exhibited excellent electrochemical performance. However, to the best of our knowledge, these nanostructures would not fully realize their electrochemical performance because the atoms in the core of the structures can not effectively take part in electrochemical reactions during the charge/discharge processes. Therefore, how to make every atom in Co₃O₄ nanostructures to take part in chemical reactions is worthy of consideration. It was reported that two-dimensional (2D) Co₃O₄ nanostructures are not only beneficial for efficient diffusion of the electrolyte and transfer of electrons, but also improve the energy output and the cycling stability.²⁵ In comparison to three-dimensional (3D) nanostructures, such as nanocubes, nanoflowers, nanotubes and microspheres, the 2D textural features, such as nanosheets and nanobelts, display a huge percentage of surface atoms and specific facet exposed, which makes full use of active materials in the electrode for LIBs. Therefore, it is suggested that Co₃O₄ with 2D architecture might be superior anode materials for LIBs. For example, we ever reported that Ni_xCo_3−xO₄ nanosheets exhibited excellent Li-storage performance and cycling stability as anode materials for LIBs.²⁶ On the basis analysis, it is desirable to design and fabricate 2D Co₃O₄ as anode materials for LIBs, which is favorable for promoting the interface contact area between electrode and electrolyte, facilitating the transfer of lithium ions and electrons, and decreasing the volume expansion during the charge/discharge processes.

In this study, we aim to develop a novel and facile strategy for the large-scale synthesis of 2D Co(OH)₂ precursor, which are transformed to porous Co₃O₄ nanobelts after the heat treatment in air. During the annealing process, the release of a large number of gaseous H₂O molecules would generate large specific surface area and huge pore volume in the as-obtained sample. The electrochemical performance of the as-obtained porous Co₃O₄ nanobelts is evaluated as anode materials for LIBs.

2. Experimental section

Materials preparation

All chemicals were of analytical grade, and were used without any further purification. In a typical procedure, 1 mmol of Co(AC)₂·2H₂O, and 0.3 g L-arginine were added into the mixed solution of 12 ml of deionized water and 6 ml of ethanol to form a transparent solution. Then, 1 ml of NH₃·H₂O was added into the above solution. After being vigorously for 10 min, the mixture was transferred into a Teflon-lined stainless steel autoclave with a capacity of 25 ml. The autoclave was sealed, maintained at 100 °C for 8 h, and cooled to room temperature. The resulted precipitates were filtered and washed several times with deionized water and absolute ethanol, respectively, and finally dried under oven at 60 °C. After calcining the collected precursor at 450 °C for 2 h with a heating rate of 1 °C min⁻¹ in air, the porous Co₃O₄ nanobelts were obtained. The sample was then ready for further characterization.

Material characterization

The powder X-ray diffraction (XRD) patterns of all samples were recorded with a X-ray diffractometer (Japan Rigaku D/MAX-γA) equipped with Cu-Kα radiation (λ = 1.54178 Å) over the 2θ range of 10–70°. Field emission scanning electron microscopy (FE-SEM) images were collected on a JEOL JSM-6700 M scanning electron microscope. Transmission electron microscopy (TEM) images were taken on Hitachi H-800 transmission electron microscope using an accelerating voltage of 200 kV, and high-resolution transmission electron microscope (HRTEM) (JEOL-2011) was operated at an acceleration voltage of 200 kV. The specific surface area was evaluated at 77 K (Micromeritics ASAP 2020) using the Brunauer–Emmett–Teller (BET) method, while the pore volume and pore size were calculated according to the Barrett–Joyner–Halenda (BJH) formula applied to the adsorption branch. Thermogravimetric analysis (TGA) was carried out using a Shimadzu-50 thermoanalyser under air flow.

Electrochemical measurements

The electrochemical behavior of as-obtained porous Co₃O₄ nanobelts was examined by using CR2032 coin-type cells with lithium serving as both the counter electrode and the reference electrode. The working electrode was prepared by compressing a mixture of the active materials, conductive material (acetylene black, ATB), and binder (polyvinylidene fluoride (PVDF)) in a weight ratio of Co₃O₄/carbon/PVDF = 8 : 1 : 1 onto a copper foil current collector and then drying at 80 °C for 12 h. The electrolyte used in the cells was 1.00 M LiPF₆ in ethylene carbonate and diethyl carbonate (EC : DEC = 1 : 1). The density of the active material is about 1 mg cm⁻² and the film thickness was about 100 μm. The cells were assembled in an argon-filled glovebox with both the moisture and the oxygen content bellow 1 ppm (Mikrouna, Super(1220/750/900)). The electrode capacity was measured by a galvanostatic discharge–charge method in the voltage range between 0.01 and 3.0 V on a battery test system (Neware CT-3008W).

3. Results and discussion

The porous 2D Co₃O₄ nanobelts were controllably synthesized through a facile solvothermal method followed by a simple post annealing process in air, as shown in Fig. 1. Firstly, the Co(OH)₂ precursor were synthesized in a mixed solution with an H₂O–EtOH volume ratio of 2. The whole synthesis process is simple without the need of any surfactant, which is considered to be very facile and suitable for large-scale synthesis. The Co²⁺ are expected to precipitate in the alkaline condition to form Co(OH)₂ precursor. At the initial stage of the reaction, L-arginine containing both –NH₂ and –COOH groups can be easily coordinated to metal ions (Co²⁺) to control the nucleation rate of the Co(OH)₂ precursor. With the reaction time prolonging, the Co(OH)₂ nanobelts were formed.²⁶ When the as-obtained precursor were further annealed at 450 °C for 2 h with a heating rate of 1 °C min⁻¹ in air, the porous Co₃O₄ nanobelts were successfully synthesized. The as-obtained Co₃O₄ nanobelts with void space can effectively relieve the strain caused by volume variation and reduce the path for the rapid transfer of lithium ions and electrons over the whole electrode during the charge/discharge process.

Fig. 1 Schematic illustration of the formation and the charge/discharge process of the as-prepared Co₃O₄ nanobelts. First, the porous Co₃O₄ nanobelts were prepared through the calcination of the Co(OH)₂ precursor. Second, the electrolyte can quickly diffusion on the surface of the porous Co₃O₄ nanobelts, and further easily diffuse into the inner part of the porous Co₃O₄ nanobelts from the surface during the charge/discharge processes. Third, the 2D porous structure can make more atoms residing on the surface, and as a result, more active sites in the electrode can be provided for the lithium electrochemical reaction. Additionally, numerous nanopores within the nanobelts can store a large number of electrons and lithium ions, which is beneficial for enhanced performance for LIBs.

Fig. 2a exhibits X-ray power diffraction (XRD) pattern of the precursor, in which all of the diffraction peaks are in a good agreement with the standard values of Co(OH)₂ (JCPDS card no. 73-1520). The sharp peaks indicate that the as-prepared sample has good crystallinity. The thermogravimetric analysis (TGA) of as-prepared Co(OH)₂ precursor was further performed in air at a heating rate of 10 °C min⁻¹ from room temperature to 800 °C and the results are shown in Fig. 2b. There is a sharp weight loss between 180 and 400 °C, and the total weight loss is approximately 15.8%, which indicates the decomposition of the as-prepared Co(OH)₂ precursor to cobalt-based oxide. The weight loss can be assigned to the removal of H₂O molecules from the Co(OH)₂ structures, which resulted in the formation of porous structure in the calcined samples. In order to ensure calcination of the precursor completely, a temperature of 450 °C is chosen as the calcination temperature for the complete conversion of the as-prepared Co(OH)₂ precursor to cobalt-based oxide.

Fig. 2 (a) XRD pattern of as-prepared Co(OH)₂ precursor. (b) Thermogravimetric analysis (TGA) of as-prepared Co(OH)₂ precursor.

As expected, the as-prepared Co(OH)₂ precursor is thoroughly transformed into porous Co₃O₄ after annealing at 450 °C in air with a heating rate of 1 °C min⁻¹. Fig. 3 shows the XRD pattern of the calcined sample, in which all of the diffraction peaks can be well indexed to spinel Co₃O₄ (JCPDS no. 42-1467, space group: Fd3m, lattice constant a = 8.08 Å). According to the patterns, no diffraction peaks of Co(OH)₂ are observed, indicating the complete thermal conversion of Co(OH)₂ to Co₃O₄. Fig. 4 exhibits typical scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the as-prepared precursor, respectively. As shown in Fig. 4, it can be clearly observed that the as-prepared Co(OH)₂ precursor are large in scale with approximately 1 μm in width and several micrometers in length, indicating that it contains belt-like morphology. The thickness of a typical nanobelt is approximately 20 nm, which is marked by white arrows in Fig. S1(a).† In addition, a compact structure with smooth surface can be clearly seen.

Fig. 3 XRD pattern of the porous Co₃O₄ nanobelts.

Fig. 4 (a, b) SEM and (c, d) TEM images at different magnifications of the as-prepared Co(OH)₂ precursor, respectively.

The morphology and structural features of the as-prepared Co₃O₄ nanobelts were further investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and energy-dispersive X-ray spectroscopy (EDS). Fig. 5a, b and S1b† exhibit low- and high-magnification SEM images of the as-prepared Co₃O₄ nanobelts, which reveals that the size and shape of the as-prepared Co₃O₄ nanobelts are consistent with the Co(OH)₂ precursor. In addition, the high-magnification SEM image of the as-prepared Co₃O₄ nanobelts (Fig. 5b) clearly exhibits that the surface of the as-prepared nanobelts is coarse. To better illustrate the structure and porosity of the as-prepared Co₃O₄ nanobelts, representative TEM images are shown in Fig. 4c and d. The as-prepared sample has 2D porous architecture, and a large number of pores within its nanobelts can be clearly seen, which is consistent with the result observed by SEM. Moreover, the formation of pores may be attributed to the impact of gas evolution during the thermal decomposition reaction from the Co(OH)₂ precursor. Fig. 5e exhibits HRTEM image of the as-prepared Co₃O₄ nanobelts, and the measured interplanar distance of a randomly selected single nanocrystal is 0.25 nm, which is in good agreement with the (311) plane of spinel Co₃O₄, thus confirming the XRD analysis. Elemental composition analysis of the as-prepared Co₃O₄ nanobelts obtained from energy-dispersive X-ray spectroscopy (EDS) indicates the existence of Co and O elements without any other impurity elements (Fig. 5f), further suggesting high purity of the resulted sample.

Fig. 5 Morphological and elemental analysis of the porous Co₃O₄ nanobelts. (a, b) SEM images, (c, d) TEM images, (e) HRTEM image and (f) EDS spectrum of the porous Co₃O₄ nanobelts.

The porous texture of the porous Co₃O₄ nanobelts was further investigated at 77 K by the N₂ adsorption–desorption isotherm. As shown in Fig. 6, the porous Co₃O₄ nanobelts exhibit a type IV nitrogen isotherm with a H3 hysteresis loop at the relative pressure of 0.7–1.0, thus suggesting the existence of a large number of mesopores in the porous Co₃O₄ nanobelts. The Brunauer–Emmett–Teller (BET) surface area of the porous Co₃O₄ nanobelts is 41.70 m² g⁻¹, which is higher than that of 2D metal oxides, such as Mn₂O₃ nanosheets (10.85 m² g⁻¹).²⁷ From the Barrett–Joyner–Halenda (BJH) pore-size distribution pattern (inset in Fig. 6), the pore size distribution is in the range of 5 to 15 nm. Additionally, the sample has pore sizes with an average diameter of 11.4 nm, which all are in the range of mesopores. Therefore, this 2D porous structure of the Co₃O₄ nanobelts can be beneficial for the electrolyte to penetrate completely into the pores and diffuse efficiently to active sites with less resistance, and also can buffers huge volume expansion during the charge/discharge processes.

Fig. 6 Nitrogen adsorption–desorption isotherm and the corresponding pore size distribution (inset) of the porous Co₃O₄ nanobelts.

Considering that the electrodes with 2D porous nanostructures are advantageous for LIBs, we investigated the electrochemical performance of the as-prepared porous Co₃O₄ nanobelts by using the standard Co₃O₄/Li half-cell configuration. Fig. 7a exhibits the first three cyclic voltammogram (CV) curves of the electrodes made from 2D porous Co₃O₄ nanobelts at room temperature between 0.0 and 3.0 V at a scan rate of 0.1 mV s⁻¹. The CV curves for the first cycle is substantially different from those of the subsequent ones, especially for the discharge branch. In the first cycle, the intense reduction peak located at around 0.75 and 1.2 V, which can be attributed to the initial reduction of cobalt ions to cobalt metal and the formation of amorphous Li₂O and solid electrolyte interface (SEI) film.^17,28–31 For anodic process, one peak is record at around 2.1 V, which can be attributed to the oxidation reactions and conversion of metallic cobalt into cobalt oxide, in good agreement with other cobalt-based anode materials (e.g. CoO).^32,33 From the second cycle, the reduction peak shifts to a higher potential of about 0.9 V,³⁴ while the oxidation peak position almost unchanged. Additionally, the intensity of the anode peak drops significantly in the subsequent cycles relative to that in the first one, indicating the occurrence of some irreversible reactions with the decomposition of SEI film.^35,36 It is noteworthy that the subsequent CV curves exhibit good reproducibility and almost overlap, indicating the good reversibility of the electrochemical reactions.

Fig. 7 Electrochemical properties of the porous Co₃O₄ nanobelts for lithium storage. (a) The first three consecutive CV curves of the electrode made from the porous Co₃O₄ nanobelts; (b) discharge–charge curves at a current density of 100 mA g⁻¹; (c) cycling performance of the porous Co₃O₄ nanobelts at a current density of 100 mA g⁻¹ in the voltage range 0.01–3.0 V vs. Li/Li⁺; (d) rate capability test for the porous Co₃O₄ nanobelts at various current densities (100–1000 mA g⁻¹).

Fig. 7b shows representative discharge/charge voltage profiles of the porous Co₃O₄ nanobelts in different cycles at a current density of 100 mA g⁻¹ between 0.01 and 3.0 V. The initial discharge curve exhibits a long potential plateau at approximately 1.1 V followed by a gradually slope to the cutoff potential of 0.01 V, which is similar to previous reports.^37,38 However, the plateaus shifted to higher potential in the subsequent cycles. The initial discharge and charge capacities are found to be 2307 and 1246 mA h g⁻¹, respectively, leading to an initial coulombic efficiency of approximately 54%. The relative low initial coulombic efficiency can be attributed to the formation of SEI film at the electrolyte interface, the decomposition of electrolyte, the organic conductive polymer, and the reduction of adsorbed impurities on Co₃O₄ surface.^39,40 This phenomenon also matches well with the CV results that the cathodic peaks are present in the first scan while absent afterward. In addition, it is clearly observed that there is a large deviation in potential between charge and discharge curves. This characteristic is commonly exists in a large number of metal oxide anodes, due to the polarization related to ion transfer during cycling processes.^41,42

Fig. 7c exhibits the cycling performance of the porous Co₃O₄ nanobelts as a function of cycle number at a current density of 100 mA g⁻¹. After ten charge/discharge cycles, the porous Co₃O₄ nanobelts exhibit excellent cycling stability upon prolong cycling and their coulombic efficiency steadily maintains at over 99%. At the end of 60^th cycle, the discharge capacity is retained at 857 mA h g⁻¹, which is much higher than that of the commercial Co₃O₄ sample (244 mA h g⁻¹). Additionally, after tested 40 cycles as an anode electrode for LIBs, it is interesting to note that the capacities of the sample display a gradual increase during the charge/discharge cycles, which is attributed to the activation of the porous structure. This characteristic is common in a large number of the porous materials and cobalt-based materials.^26,28 The rate capability was evaluated at continuously varying current densities from 100 to 1600 mA g⁻¹. As shown in Fig. 7d, as the current density increased from 100 to 200, 400, 800 and 1600 mA g⁻¹, the capacity decreases only slightly from 726 to 585, 443, 292 and 167 mA h g⁻¹, respectively. When the current density was reduced from 1600 to 100 mA g⁻¹, a capacity of 845 mA h g⁻¹ can be recovered, which also has a trend to increase. The porous Co₃O₄ nanobelts exhibit excellent rate capability as anode materials for LIBs. To investigate structural integrity of the Co₃O₄ anode after 60 cycles, a battery was disassembled and examined by SEM and TEM, respectively. As shown in Fig. S2,† the original belt-like of Co₃O₄ is not clearly observed, because the active materials were covered by a SEI film and the mixtures of PVDF and acetylene black. Additionally, pores in the porous nanobelts are filled presumably by the residual Li₂O or SEI materials, indicating that pores provide a secondary expansion pathway. This phenomenon further demonstrates that the as-prepared Co₃O₄ nanobelts are ideal candidates for next generation high performance LIBs.

Based on above experimental results, the excellent electrochemical performance of the as-prepared porous Co₃O₄ nanobelts can be attributed to their unique 2D porous structure with a variety features (Fig. 1). First, the as-prepared 2D nanobelts with high specific surface area enlarge the interfacial contact area with the electrolyte and buffer the volume variation during cycling processes. The 2D nanobelts are also favorable for shortening the pathway for both Li⁺ diffusion and electron transfer and thus significantly enhances their capacity retention. Second, the 2D porous nanobelts are not only beneficial for specific facet exposure, in which lithium insertion is just like surface lithium storage, but also are less prone to structural collapse during the charge/discharge processes, which is likely to be the reason why good rate capability and cycling stability are achievable.^25,26 Last, nanopores within 2D nanobelts can store Li⁺ and electron during the charge/discharge processes, which provides an extra capacity for the as-prepared Co₃O₄ nanobelts.⁴ Therefore, the 2D porous Co₃O₄ nanobelts can provide a carrier for the penetration of the electrolyte and transport of lithium ions and electrons into electrode, and subsequently improve the lithium storage performance efficiently.

4. Conclusions

In summary, we have developed a facile and efficient method to prepare the 2D porous Co₃O₄ nanobelts by decomposition of Co(OH)₂ precursor at 450 °C in air. The as-prepared porous Co₃O₄ nanobelts possess high surface area and comprise numerous nanopores within their nanobelts. Also, these 2D porous Co₃O₄ nanobelts display a huge percentage of surface atoms and specific facet exposed, which permits a high interfacial contact area with the electrolyte and facilitates electrochemical reactions. Benefiting from their unique structural features, the as-prepared Co₃O₄ nanobelts not only facilitates the fast transport of lithium ions and electrons but also alleviates the volume expansion during the discharge/charge processes. When tested as an anode material, the porous Co₃O₄ nanobelts can retain a reversible capacity of 857 mA h g⁻¹ at 100 mA g⁻¹ after 60 cycles, thus indicating that the porous Co₃O₄ nanobelts has a potential as an anode material for LIBs. In addition, this work may provide a novel route for the large-scale fabrication of other 2D porous structures for LIBs.

Acknowledgements

This work was supported by Science and Technology Commission of Shanghai Municipality (No: 14DZ2261000), Anhui Provincial Natural Foundation (No. 1608085QB34) and the National Natural Science Foundation (NSFC, 21371009).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra23835h

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