Enhancing the electrode performance of Co₃O₄ through Co₃O₄@a-TiO₂ core–shell microcubes with controllable pore size

Abstract

Co₃O₄ is a potential high-capacity anode material for lithium ion batteries (LIBs), but it usually has poor cycling stability due to its enormous volume variation during the conversion reaction process. A protection strategy has been developed to enhance the electrode performance of Co₃O₄ through construction of Co₃O₄@amorphous-TiO₂ (Co₃O₄@a-TiO₂) core–shell nanostructures with controllable pore size, realized by precisely regulating the volume ratio of ethanol to water. To our knowledge, this is the first time to use core–shell Co₃O₄@a-TiO₂ as the anode material for the lithium-ion batteries. Noteworthy, electrochemical performances of the materials were found to be strongly correlated with their porous structures. With the excellent stability of TiO₂ sheath, high capacity of Co₃O₄ core, and the optimized porous size, these unique Co₃O₄@a-TiO₂ core–shell microcubes exhibit high capacity, long cycle life and good rate capability as advanced electrode materials for lithium-ion batteries. A high reversible capacity of ∼800 mA h g⁻¹ is obtained at 500 mA g⁻¹, keeping the value stable after 60 cycles. We anticipate that such a demonstration presents a versatile method toward synthesizing other high-performance and multifunctional metal oxides@a-TiO₂ nanocomposites for LIBs in near future.

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1. Introduction

Lithium-ion batteries (LIBs) are one of the most promising energy storage devices for their advantages, including long lifetime, high capacities and energy density. However, restricted by some disadvantages, the present electrode materials could not satisfy the requirements of high-energy applications such as portable electronics, electric vehicles (EVs) and hybrid electric vehicles (HEVs). Recently, transitional metal oxides (TMOs)-based nanostructured materials have been widely used as negative electrode materials for LIBs due to high specific capacity (Fe₂O₃, 1007 mA h g⁻¹; MnO₂, 1232 mA h g⁻¹), which is two or three times higher than that of currently used graphite anode (≈370 mA h g⁻¹). Cobalt oxides (mainly Co₃O₄ and CoO), among the alternative anode candidates available, have been judiciously selected due to low cost synthesis and their much higher capacity than that of conventional graphite.^1–4 In the scenario, lithium storage is mainly relied on the reversible conversion between lithium ions and oxide anode accompanied by formation of metallic nanocrystals dispersing in Li₂O matrix. During this process, capacity usually degrades in a fast rate caused by noticeable volume expansion and consequential destruction of the electrode. Moreover, the intrinsic drawback of low electronic conductivity also, to some extent, contributes to performance degradation in metal oxide involved electrodes, especially at high current densities. To circumvent the problems, development of cobalt oxide based micro/nanostructures engineered with optimized particle sizes, configuration and component compositions, had been made.^5,6 Despite dramatic endeavors to exploit the field of anode materials, how to fabricate truly durable cobalt oxide electrodes remains a noteworthy challenge with desired rate capability and specific capacity.

Recently, hybridization of nanostructures, consisting of several components, has been emerging as one of the most feasible means towards high-performance materials for LIBs. In this regard, different motifs are explored with active materials either embedded into or assembled onto the conductive matrix by virtue of chemical bonding or various noncovalent interactions. The point of such hybrid structures is to induce strong synergetic effect obtained via integrating individual components, not existing before hybridization, besides the previously-existent functions of each constituent. As a consequence, novel functionalities can be realized by the full potential of composite materials. In fact, this concept affords the versatility in no matter engineering or science fields, wherein recent achievements have been promoted such as Fe₂O₃/SnO₂, Fe₂O₃/TiO₂, Fe₂O₃/ZnO, TiO₂/SnO₂, MnO₂/Fe₂O₃, and so on.^7–18 Amongst them, titanium dioxide (TiO₂) shows nice adaptability to different applications considering its intrinsic properties such as good chemical and physical stability, low cost, environmental friendliness, and plentiful polymorphs.¹⁹ Hence, the integration with TiO₂ offers a promising possibility to improve the LIBs capacity and cycling stability. It should be mentioned that several polymorphs of TiO₂, including rutile,²⁰ anatase²¹ and brookite,^22,23 and even amorphous,²⁴ have been investigated for LIBs. Most reports focus on the study of abundant morphologies of anatase and rutile whereas few are pertain to amorphous TiO₂.

Stimulated by the above considerations, herein, we proposed a convenient approach by combining solvothermal and sol–gel processes to construct 3D porous Co₃O₄@a-TiO₂ core–shell micro/nanostructures. More importantly, by adjusting the solvent composition in the sol–gel step, the shell-TiO₂ with controllable pore size was obtained (see Fig. 1). Such a novel hybrid micro/nanostructure is expected to confirm remarkably enhanced electrochemical performance due to the incorporation of favorable structural features. To be specific, excellent stability of TiO₂ sheath, remarkable capacity of Co₃O₄ core, and the appropriate pore size distribution are favorable for the stability of electrode materials during the discharge–charge process. When served as the anode material for LIBs, 3D porous Co₃O₄@a-TiO₂ core–shell architectures with optimal pore size indicate remarkable electrochemical performance. A high reversible capacity of ∼800 mA h g⁻¹ was reached and kept stable at 500 mA g⁻¹.

Fig. 1 Schematic illustration for the preparation of the S-40, S-80, S-120.

2. Experimental section

Synthesis of Co₃O₄ microcubes

All of the reagents were purchased and used without further purification. CoCO₃ micro/nanoparticles were prepared according to our previous modified method with several variation.²⁵ In a typical synthesis, 1.5 mmol of Co(CH₃COO)₂·4H₂O and 30 mmol NH₄HCO₃ were dissolved in 40 mL HOC₂H₄OH under stirring. The as-obtained homogeneous mixed solution was transferred into 60 mL Teflon-lined stainless steel autoclave after stirring for 1 h and heated at 200 °C for 20 h. The obtained pink precipitation were washed for three times by a ethanol–water mixture, and dried in air at 80 °C overnight. The CoCO₃ precursors were converted to black Co₃O₄ powers after annealing at 300 °C for 4 h in air.

Synthesis of Co₃O₄@a-TiO₂ composites

Typically, about 0.1 g of the Co₃O₄ microcubes were well dispersed in ethanol (40 mL) and deionized water (0.3 mL) under stirring for 30 min and ultrasonication for 15 min at room temperature. After adding 0.5 mL tetrabutyl orthotitanate (TBOT) in ethanol (3 mL) dropwise, the solution was heated to 80 °C in an oil bath for 100 min. The product was collected through centrifugation and washed with ethanol–water mixture for several times. Eventually, the collection was dried in a vacuum oven at 80 °C. In order to obtain the other two products, we can alter the volume of the ethanol with the other experiment conditions unchanged.

Materials characterization

SEM images were obtained with a field-emission scanning electron microscope (FESEM, JSM-6700F, JEOL) operated at 5 kV. High-resolution TEM (HRTEM), scanning TEM, and energy dispersive X-ray (EDX) analysis were collected on a JEM-2100F (JEOL) instruments operating at 200 kV accelerating voltage. X-Ray photoelectron spectra (XPS) measurements were conducted with an X-ray photoelectron spectroscopy (XPS, VGESCA-LABMK II spectrometer, (ESCALAB 250) using a twin-anode Al K_α (1486.6 eV) X-ray source). Power X-ray diffraction (PXRD) characterization was carried out using a Bruker-D8 Advanced X-ray diffractometer with a Cu Kα radiation source and a wavelength of 1.5406 Å. The N₂ adsorption–desorption isotherms were obtained from a Micromeritics analyzer (ASAP 2020, USA) at 77 K.

Electrochemical measurements

Electrochemical experiments were performed in 2032 coin type cells. The working electrodes were prepared by mixing the samples (Co₃O₄ or Co₃O₄@a-TiO₂), acetylene black, and polymer binder (sodium carboxymethyl cellulose, CMC) at a weight ratio of 70 : 20 : 10 and pasted on pure copper foil and the thickness of the coating is 200 μm. After drying under vacuum at 80 °C for 12 h, the resulting foil was roll-pressed and cut into 12 mm discs in diameter, making the density of the active material is about 1 mg cm⁻². Lithium foil was used as both the counter electrode and reference electrode, and the Celgard 2300 microporous polypropylene membrane was used as the separator. The electrolyte consisting of a solution of 1 M LiPF₆ in ethylene carbonate (EC), dimethylcarbonate (DMC) and diethyl carbonate (DEC) (1 : 1 : 1, v/v/v) was obtained from Samsung Chemical Ltd. The cells were assembled in an argon-filled glovebox (Mikrouna, Super 1220/750/900) with both the moisture and the oxygen content below 1 ppm. The galvanostatic charge–discharge measurements were performed using CT2001A LAND Cell test system at various current densities. The cyclic voltammetry (CV) was conducted at 0.1 mV s⁻¹ with the range of 0.01–3.5 V on a LK2005A electrochemical workstation.

3. Results and discussion

Uniform Co₃O₄ microcubes (noted as S-0) synthesized through a simple solvothermal and subsequent calcination are employed as the cores. As revealed by FESEM, these Co₃O₄ microcubes are uniform with an average diameter at ∼1 μm (Fig. 2a). Then, through a sol–gel process and adjusting the volume of the solvent, 3D core–shell Co₃O₄@a-TiO₂ microcomposites with controllable pore size were synthesized (details see Experimental section), as displayed in Fig. 2b–d. When the solvent is 40 mL, the samples (noted as S-40) have the rough surfaces and consists of the small and tight nanoparticles (see Fig. 2b). As the volume of the ethanol increasing to 80 mL, the bigger and less tight a-TiO₂ units were around the core Co₃O₄, forming another Co₃O₄@a-TiO₂ samples (noted as S-80) (Fig. 2c). As shown in Fig. 2d, the surface a-TiO₂ particles of the S-120 samples are the biggest and loosest, indicating the biggest pore size. The formation of the a-TiO₂ layer is also supported by high-magnification TEM images. As described in Fig. S1,† the S-40, S-80 and S-120 show the a-TiO₂ layer thickness of 73, 48 and 32 nm.

Fig. 2 Typical FESEM images of Co₃O₄ (S-0: a) and Co₃O₄@a-TiO₂ (S-40: b; S-80: c; S-120: d).

On the basis of the formation mechanisms of Zhao's group,²⁶ we propose a approximate coating process. When the addition of a small volume (40 mL) of ethanol into the system, the concentration of the titanium oligomers is high, resulting in making a-TiO₂ shells in thickness. Upon increasing the volume of ethanol to 80 mL, making a lower concentration of the titanium oligomers, a thin a-TiO₂ layers were formed on the surface of the Co₃O₄ nanoparticles. Finally, the thinnest a-TiO₂ networks are formed. With a high volume of ethanol (120 mL), the concentration of the titanium oligomers is very low; thus there is only a very small amount of a-TiO₂ coated on Co₃O₄. The formation process is indicated schematically in Fig. 1.

The morphology feature can further be elucidated from representative TEM image that the diameter of the cubic like S-0 is around 1 μm, as indicated in Fig. 3a. Low-magnification TEM image of the S-80 described in Fig. 3b shows that a-TiO₂ nanoparticles are strongly anchored on the Co₃O₄, implying the formation of the Co₃O₄@a-TiO₂. Fig. 3d indicates a locally magnified HRTEM image of core Co₃O₄ recorded from Fig. 3c. The 0.24 nm can be assigned to the (311) and (3−1−1) interplane spacings with zone axis of [0−11] parallel to incident electron direction. Conversely, no evident lattice fringes can be detected in Fig. 3c (the edge of the S-80 shown by white arrows), revealing that Co₃O₄ cores are well crystallized and TiO₂ shells are amorphous. To indicate the phase structure of the samples, X-ray diffraction (XRD) measurements were performed. Fig. 3e presents XRD pattern of the S-0, S-40. The diffraction peaks of S-0 prepared by the pyrolysis of CoCO₃, within the experimental error, can be assigned to cubic Co₃O₄ (a₀ = 8.056 Å; space group Fd3̄m (227), JCPDS Card no. 65-3103). Compared with the peaks of S-0, there are no difference in the pattern of S-40, indicating the presence of a-TiO₂. In order to further illustrate the phase of the shell TiO₂, the sample prepared without Co₃O₄ were tested. As shown in Fig. S2,† no crystal-clear diffraction peaks corresponding to the crystalline phase are detected, further revealing the amorphous structure of TiO₂ in the three samples (S-40, S-80, S-120). Energy dispersive X-ray spectroscopy (EDS) maps clearly showed the distribution of Co, Ti, and O are quite uniform throughout the whole S-80, which demonstrates the formation of the a-TiO₂.

Fig. 3 (a) TEM of S-0. (b–d) TEM and HRTEM of S-80. (e) STEM image of S-80 and the corresponding elemental mappings for Co, Ti, O. (f) XRD pattern of the S-0, S-40.

EDS presents the element analysis of the S-40, S-80, S-120 (Fig. S3†) and Co, Ti and O elements are detected in the three samples, indicating the coexistence of Co₃O₄ and a-TiO₂. The Co/Ti ratio is further analyzed and confirmed by ICP-AES (Table S1†). The results further show the presence of the Co and Ti elements in the S-40, S-80, S-120. To further investigate the more detail elemental composition and the oxidation state of the Co₃O₄@a-TiO₂ microcubes (S-40), we also carried out an X-ray photoelectron spectroscopy (XPS) measurements and the corresponding results are shown in Fig. 4. All of the binding energies (BEs) in this XPS analysis were corrected for specimen charging by referencing them to the C 1s peak (set at 284.6 eV). The survey spectrum (Fig. 4a) indicates the presence of the Co, Ti and O as well as C from the reference and the absence of other impurities, which is in good agreement with the EDS results. The high-resolution spectrum for the O 1s region (Fig. 4b) shows three oxygen contributions. The peaks at 529.2 eV and 530.0 eV are commonly assigned to the oxygen species of the Co–O and Ti–O bonds in the Co₃O₄ and TiO₂.^27–30 The peak sitting at 531.7 eV is usually ascribed to defects contaminants, and numerous surface species including hydroxyls, the oxygen in the physi- and chemi-sorbed water or under coordinated lattice.^31–33 The Co 2p spectrum (Fig. 4c) indicates two major peaks with BE values at 779.6 and 794.7 eV, attributed to the Co 2p_3/2 and Co 2p_1/2 peaks, respectively, with a spin–orbit splitting of 15.1 eV.^34,35 The absence of prominent shake-up satellite peaks in the Co 2p spectra further demonstrates the formation of the Co₃O₄ phase.^2,36,37 The Ti 2p spectrum in Fig. 4d features two main spin–orbit lines of Ti 2p_1/2 at 464.5 eV and Ti 2p_3/2 at 458.6 eV, indicating the dominant Ti(IV). Furthermore, a peak separation of 5.9 eV is noticed between the two Ti 2p peaks, which is in good agreement with values reported in the literature.³⁸ It is reasonable, therefore, to confirm that the composites consist of the core Co₃O₄ and the shell a-TiO₂.

Fig. 4 XPS spectra of (a) survey spectrum, (b) O 1s, (c) Co 2p, and (d) Ti 2p for Co₃O₄@a-TiO₂ (S-40).

In order to verify the porous structure and the Brunauer–Emmett–Teller (BET) surface areas, the four samples are examined by N₂ sorption at 77 K (Fig. 5). As shown in the Fig. 5a, a distinct hysteresis loop can be classified as type IV with a type H1 hysteresis loop, indicating the mesoporous structures. According to the corresponding Barrett–Joyner–Halenda (BJH) plots (Fig. 5b) recorded from nitrogen sorption of the S-0, S-40, S-80 and S-120 samples, the pore size distribution is in the 2–50 nm range and shows the domination of 14.8, 12.6, 11.6, 16.9 nm mesopores, respectively. Surface areas calculated by BET method are shown in Table S2,† indicating that specific surface areas were around 73.55, 105.67, 85.40, 56.38 m² g⁻¹ for S-0, S-40, S-80 and S-120 samples, respectively. While the volume of the ethanol is increased from 40 to 120 mL, the average pore size of the three Co₃O₄@a-TiO₂ composites can be regulated from 5.2 to 10.87 nm, which is consistent with the observation of FESEM. Accordingly, the pore volumes of the Co₃O₄@a-TiO₂ composites can also be regulated in the range from 0.1397 to 0.2088 cm³ g⁻¹. The average pore size and pore volume of the S-0 samples are 9.49 nm and 0.2486 cm³ g⁻¹, respectively.

Fig. 5 Nitrogen adsorption–desorption isotherms for the four samples (a) and the corresponding pore size distributions (b).

Fig. 6a depicts the first five cyclic voltammetry (CV) curves of the S-80 electrodes between 0.01–3.5 V (vs. Li/Li⁺) with a scan rate of 0.1 mV s⁻¹. In the first cathodic scan, the cathodic peak at 1.0 V was observed, which correspond to the insertion process of the Li⁺ in the a-TiO₂. It can be clarified as follows: TiO₂ + xLi⁺ + xe⁻ → Li_xTiO₂. The strong peak at around 0.8 V is assigned to the formation of a solid electrolyte interface (SEI) layer and the reduction of Co₃O₄,^1,39 which can be described by Co₃O₄ +4Li⁺ + 4e⁻ → Co + 2Li₂O. There are two broad oxidation peaks at 1.3 V and 2.1 V in the first anodic process. The peak at 1.3 V usually is ascribed to the decomposition of the SEI layer and the peak at 2.1 V is allocated to the deinsertion of Li⁺ in the Li_xTiO₂ and oxidation of Co to Co₃O₄. The second and onward CV curves remain overlapped, showing the excellent cycle stability of the S-80. The CV measurement is carried out to evaluated the electrochemical behavior of the Co₃O₄, as shown in Fig. S4,† which corresponds to the redox of Co/Co₃O₄ and the formation/decomposition of the SEI.

Fig. 6 (a) Cyclic voltammetry (CV) curves for the S-80 at a scan rate of 0.1 mV s⁻¹ in the voltage windows of 0.01–3.5 V. (b) Charge–discharge curves of S-80 at a current density of 0.5 A g⁻¹. (c) Cycling performance of S-0, S-40, S-80, S-120 at a current of 0.5 A g⁻¹. (d) Cycle performance of S-0, S-40, S-80, S-120 at various current densities.

The electrochemical performance of the S-80 microcubes have been further investigated by galvanostatic discharge–charge tests at 0.5 A g⁻¹ between 0.01 and 3.5 V, which is also consistent with that obtained from CV measurements, as shown in Fig. 6b. The voltage profiles of Co₃O₄@a-TiO₂ composites (S-80) in the first cycle presented a slope between 1.7–0.9 V, which should be according to the amorphous TiO₂. An inclined plateau between 0.9–0.7 V associated with the conversion from Co₃O₄ to an intermediate-phase Li_xCo₃O₄ and then to Co metal, followed by a sloping curve down to the voltage of 0.01 V, which is typical for the Co₃O₄ electrode. It is observed that the initial discharge and charge capacities are 1100 and 790 mA h g⁻¹ for the Co₃O₄@a-TiO₂ electrode, the initial capacity loss may result from the insufficient conversion reaction and irreversible lithium loss because of the formation of solid electrolyte interphase (SEI) film. The Coulombic efficiency (CE) of Co₃O₄@a-TiO₂ composite rapidly rises to about 100% in the second one and then remains in the following cycles, showing an excellent reversibility.

To highlight the superiority of the S-80 for anode material of LIBs, the cycle performance of the S-80 was tested at a current 0.5 A g⁻¹ for 60 cycles in the voltage of 0.01 to 3.5 V. For comparison, S-0, S-40, S-120 were also investigated under the same conditions. It is found that the capacity of the S-0 decays with cycling, and S-120 also shows a decreasing capacity when the capacity reaches its maximum value (Fig. 6c). It is supposed that no or too small amount of a-TiO₂ restrict the cycling stability. The reversible capacities of S-40 and S-80 remain stable during 60 cycles. The capacity of the S-80 electrode, 800 mA h g⁻¹, is the highest after 60 cycles, which can be attributed to the suitable amount of a-TiO₂ and higher specific surface area. As expected, the S-80 electrode also exhibits good rate capacity, as displayed in Fig. 6d. S-40 displays the relatively lower specific discharge capacity at the current densities of 0.1–2 A g⁻¹. The results show that too many a-TiO₂ cause a decrease of the capacity. The corresponding CE of the cycling curves of four samples were supplied, as shown in Fig. S5.† For examples, their CE of S-80 quickly increases from 65% for the first cycle to about 99% after three cycles and remains nearly 100% thereafter, which reveals a convenient lithium insertion/extraction associated with efficient transport of ions and electrons in the electrodes. To further confirm the long cycling stability of the S-80 at higher current rate, the electrode was conducted at 2 A g⁻¹ for 50 cycles as shown in Fig. S6.† It is noteworthy that the discharge capacity can retain a value as high as 400 mA h g⁻¹ after more than 50 cycles, which is still much higher than the theoretical capacity of graphite (370 mA h g⁻¹). To further understand the structural stability of the Co₃O₄@a-TiO₂ composites (S-80), we studied the SEM and TEM after 20 discharge–charge cycles at 0.5 A g⁻¹ (see the Fig. 7). As seen from the SEM and TEM images, some Co₃O₄@a-TiO₂ porous microcubes still can be evident, suggesting its desirable structural stability over cycling, which could contribute greatly to the good cycling stability. For comparison, the cycling performance of pure a-TiO₂ nanoparticles were tested, as shown in Fig. S7.† The reversible capacity of less than 200 mA h g⁻¹ after the first cycle was sustained, confirming that TiO₂ nanoparticles contributed very little to the capacity. It is noteworthy that the CE is very close to 100% all over the 200 cycles. These results suggest the a-TiO₂ maintains extraordinarily stability and is in favor of the improvement of electrochemical performance of our composite. Thus the electrochemical performance of the materials can be effectively controlled by the pore size and the amount of the a-TiO₂.

Fig. 7 TEM (a) and SEM (b) images of the S-80 electrode after 20 charge–discharge cycles at current of 0.5 A g⁻¹.

4. Conclusion

In conclusion, an advanced Co₃O₄@amorphous-TiO₂ (Co₃O₄@a-TiO₂) core–shell nanostructures have been constructed through combining solvothermal and subsequent sol–gel processes. More importantly, through adjusting the solvent composition in the sol–gel step, the shell-TiO₂ with controllable pore size was obtained. The as-obtained heterostructures combine both merits from TiO₂ such as good cycle stability and Co₃O₄ with high capacity. With the excellent stability of TiO₂ sheath, high capacity of Co₃O₄ core, the optimized porous size, and the synergetic effect between Co₃O₄ and TiO₂, these unique Co₃O₄@a-TiO₂ core–shell microcubes (S-80) exhibit long cycle life, high capacity, and excellent rate capability as advanced electrode materials for LIBs. We anticipate that such a demonstration indicates a versatile method toward preparing other high-performance and multifunctional metal oxides@a-TiO₂ nanocomposites for LIBs in near future.

Acknowledgements

The authors gratefully acknowledge the financial supports provided by the National Basic Research Program of China (the 973 Project of China, Grant 2011CB935901), National Natural Science Fund of China (Grants 21371108, 21203110), Shandong Provincial Natural Science Foundation for Distinguished Young Scholar (Grant JQ201304), the National Science Foundation of Shandong Province (Grant ZR2012BM018), and the Opening Project of CAS Key Laboratory of Materials for Energy Conversion (Grant KF2014002).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra06711a

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