Hierarchically ordered mesoporous Co3O4 materials for high performance Li-ion batteries

Highly ordered mesoporous Co3O4 materials have been prepared via a nanocasting route with three-dimensional KIT-6 and two-dimensional SBA-15 ordered mesoporous silicas as templates and Co(NO3)2 · 6H2O as precursor. Through changing the hydrothermal treating temperature of the silica template, ordered mesoporous Co3O4 materials with hierarchical structures have been developed. The larger pores around 10 nm provide an efficient transport for Li ions, while the smaller pores between 3–5 nm offer large electrochemically active areas. Electrochemical impedance analysis proves that the hierarchical structure contributes to a lower charge transfer resistance in the mesoporous Co3O4 electrode than the mono-sized structure. High reversible capacities around 1141 mAh g−1 of the hierarchically mesoporous Co3O4 materials are obtained, implying their potential applications for high performance Li-ion batteries.


Results and Discussion
Material characterization. Low-angle XRD patterns indicate that all the samples are ordered mesoporous (Fig. 1a). For the products nanocast from KIT-6, both Co 3 O 4 -KIT-6-100 and Co 3 O 4 -KIT-6-130 exhibit one well-defined diffraction peak indexed as (211). They possess the same mesoscopic symmetry as their parent silicas with space group Ia3d, indicating that the mesostructures of their parent silicas were duplicated. The other two products (Co 3 O 4 -KIT-6-40 and Co 3 O 4 -KIT-6-80) exhibit two well-defined diffraction peaks indexed as (110) and (211) with space group I4 1 32 18 . Their mesoscopic symmetries are lower than those of their parent silicas. For Co 3 O 4 -SBA-15-100, a relatively small peak was displayed, which can be indexed as (100) with space group of P6mm. Hence, Co 3 O 4 -SBA-15-100 possesses the same 2D hexagonal symmetry as its template. Wide-angle powder X-ray diffraction results (Fig. 1b) show that all phases are coincident, demonstrating that the face centered cubic spinel structure dominates the wall of the mesoporous solid.
All the samples were analyzed by Transmission electron microscopy (TEM), which confirmed the highly ordered mesoporous structure (Fig. 2). Conventional cobalt oxide particles were not observed for all the samples from TEM observation. This indicates that almost all nitrates have moved into the mesopores of silicas during the calcination. Figure 2a-h shows the TEM images with different magnifications of the mesoporous Co 3 O 4 materials nanocast from KIT-6. The square image contrast pattern of Co 3 O 4 -KIT-6-40 (Fig. 2b), where the mesoporous channels are seen as bright contrast, indicating the image is viewed down the [100] zone axis of KIT-6 related cubic unit cell. Most particles of mesoporous Co 3 O 4 -KIT-6-80 (Fig. 2c) were spherical in shape with a particle size ranging from 0.64 to 1.30 μ m, indicating the crystal growth in a 3D mesoporous system. The TEM images in Fig. 2f, g are viewed along the [111] and [311] zone axis of KIT-6 related cubic unit cell, respectively 19 . Figure 2i-l shows the TEM images and the corresponding selected area electron diffraction (SAED) of the mesoporous Co 3 O 4 -SBA-15-100 material, which exhibits a worm-like overall morphology (Fig. 2i). A magnified view of a mesoporous Co 3 O 4 bundle (Fig. 2j) shows the presence of mono-dimensional aligned channels between two aligned nanorods. According to the literature 20 , adjacent Co 3 O 4 nanorods are connected by Co 3 O 4 spacers formed inside SBA-15 micropores. The SAED pattern of the area marked with a circle in Fig. 2j is shown in Fig. 2k; the ring-like diffraction pattern indicates the nanocrystalline walls of the mesoporous Co 3 O 4 -SBA-15-100. Fast Fourier transform (FFT) pattern in Fig. 2l is simply an inverse form of the entire nanowire bundle, in which the spots reflect the highly ordered arrangement of parallel nanowires. Energy-dispersive X-ray (EDX) spectra of all Scientific RepoRts | 6:19564 | DOI: 10.1038/srep19564 the mesoporous Co 3 O 4 materials confirm no trace of Si, which means that the silica templates have been completely removed. Figure 3 shows the N 2 adsorption-desorption isotherms and (Barrett-Joyner-Halenda) pore size distribution plots of mesoporous Co 3 O 4 . Typical IV adsorption-desorption isotherms with H1-type hysteresis are observed for all the samples. This is ascribed to the formation of mesoporosity. Moreover, the capillary condensation range is broad for all the sorption isotherms starting at about P/P 0 = 0.4 and extending almost to P/P 0 = 0.9. This indicates that all the samples have a high fraction of textural porosity 21 . The BJH pore size distributions show that mesoporous Co 3 O 4 -KIT-6-40 and Co 3 O 4 -KIT-6-80 have a bimodal pore-size distribution, which are centered at 5.3/10 nm and 3.5/10.9 nm, respectively. The smaller pore size of 5.3 or 3.5 nm reflects the minimum wall thickness of KIT-6, while larger pore size of 10 or 10.9 nm is corresponding to the wall junctions in KIT-6 22 . Whereas, the other two mesoporous Co 3 O 4 materials have a unimodal pore-size distribution, with the pore size of 3.5 nm for Co 3 O 4 -KIT-6-100 and 3.9 nm for Co 3 O 4 -KIT-6-130. It is well known that KIT-6 possesses two sets of mesoporous systems, which are connected by micropores. The amount of micropores depends on the temperature of hydrothermal treatment. When KIT-6 was treated at lower temperature such as 40 or 80 °C, a part of the two mesoporous systems were not connected. Accordingly, hierarchically porous structure was obtained; While KIT-6 was treated at higher temperature such as 100 or 130 °C, the two mesoporous systems were well interconnected. Accordingly, porous structure with mono-sized pores was obtained. Textural properties of these samples were summarized in Table 1 Electrochemical properties. Figure   130, only one reduction peak emerged; this is because the two reduction peaks which should appear merge together. During the following anodic polarization, one broad hump at around 1.5 V and one sharp peak at around 2.1 V were observed for all the mesoporous Co 3 O 4 , which is corresponding to the reverse process where Co is reoxidized to Co 3 O 4 and Li 2 O is decomposed 23 . Furthermore, besides the redox peaks, a rectangular shape area related to the reflection by supercapacitor 24,25 is observed at the lower potential in each CV pattern. This indicates that besides the lithium storage according to the conversion reaction of between Co 3 O 4 and lithium, the electrochemical process by the capacitive contribution is also included. Figure 5 shows the first three charge (delithiation) and discharge (lithiation) curves of ordered mesoporous     Figure 6a shows the variation of discharge capacities versus cycle number for the ordered mesoporous Co 3 O 4 electrodes cycled between 0.01-3.0 V at the current density of 50 mA g −1 . For all of the Co 3 O 4 electrodes, they demonstrate superior cycling stability. The discharge capacity gradually increases upon initial cycles, especially for the Co 3 O 4 -KIT-40 and Co 3 O 4 -KIT-6-80 with hierarchically mesoporous structure. Similar phenomenon has been also observed on Co 3 O 4 nanomaterials 26-30 . We could not explicitly explain this phenomenon. The higher surface areas of our mesoporous materials might be responsible for this behavior. The electrolyte needs some time to access the inner surface within the mesopores to establish stable electric double layer. Hence, the gradual formation of the electric double layer in the mesopores could be the reason. Furthermore, the following two points can be drawn from Fig. 6a. Firstly, the hierarchically mesoporous Co 3 O 4 -KIT-6-40 and Co 3 O 4 -KIT-80 deliver higher discharge capacities than the mesoporous Co 3 O 4 -KIT-6-100 and Co 3 O 4 -KIT-6-130 with mono-sized pores throughout the 25 cycles. We ascribe the better Li storage properties to their relatively larger BET surface areas, pore volumes and the presence of additional large pores around 10 nm, which are favorable for Li ion transport 31 (Table 1). This implies that the 3D cubic Ia3d mesoporous structure makes the infiltration of the liquid electrolyte more facile than the 2D hexagonal P6mm mesoporous structure. Besides, coulombic efficiencies are evaluated and shown in Fig. 6b. For all of the ordered mesoporous Co 3 O 4 , except for the relatively low initial coulombic efficiencies (67.9-91.1%) typical for conversion reaction 32 , the coulombic efficiencies in the subsequent cycles almost maintain above 95%, indicating their excellent electrochemical reversibility. The first discharge capacities together with those after 25 cycles for these mesoporous Co 3 O 4 electrodes are given in Table 2. The as-prepared Co 3 O 4 materials deliver high initial discharge capacities between 852-1489 mAh g −1 . After 25 cycles, the discharge capacities still maintain at a high level of 774-1141 mAh g −1 . Note that these mesoporous Co 3 O 4 electrodes exhibit capacities higher than the theoretical capacity of Co 3 O 4 (890 mAh g −1 ). This phenomenon is very common for Co 3 O 4 nanostructure 4 . These large excess capacities could be ascribed to lithium storage in the interconnected mesopores via an electric double layer capacitive mechanism, showing sloping discharge profiles at low potential in Fig. 5. Meanwhile, a rough performance comparison with other forms of Co 3 O 4 nanostructures reported before was summarized in Table 2. The as-prepared mesoporous Co 3 O 4 electrodes show comparable and/or even superior Li storage performance, which could be ascribed to their hierarchically ordered mesoporous structures. It has been demonstrated that the large surface area of the ordered mesoporous electrodes can decrease the current density per unit surface area, and the thin wall of ordered mesoporous electrodes can reduce the length of the Li + diffusion path. Moreover, compared with conventional mesoporous materials in which the pores are randomly connected, the well-ordered mesoporous materials can facilitate ionic motion more easily 11 . Most importantly, the hierarchical structure provides not only efficient transport channels for Li ions but also large electrochemically interface. Hence, the current hierarchically mesoporous Co 3 O 4 could be the choice of anode material for Li-ion batteries.
In order to account for the different electrochemical behaviors of the as-prepared ordered mesoporous Besides, for all the Co 3 O 4 electrodes, a slopping line was found in the low-frequency region. In order to interpret the measured results, an equivalent circuit model (Fig. 7b) was used to fit the Nyquist plots. The diameter and intercept of the semicircle at the Z' axis in the high-frequency region represent charge transfer resistance (Rct) and electrolyte resistance (Rs), respectively, among which Rct accounts for a large proportion of the overall Hence, ordered mesoporous Co 3 O 4 with hierarchical structure is more favorable for Li ion transport, which is consistent with the discussion above. Besides, ordered mesoporous Co 3 O 4 -KIT-6-100 exhibits smaller Rct than Co 3 O 4 -SBA-15-100, which further confirms the 3D cubic Ia3d mesoporous structure makes the transport of Li ion more facile than the 2D hexagonal P6mm mesoporous structure.

Conclusion
Textural parameters of ordered mesoporous Co 3 O 4 can be regulated by varying the hydrothermal treating temperatures of the KIT-6 template. When KIT-6 hydrothermally treated at a lower temperature of 40 °C or 80 °C was employed as the template, well-ordered mesoporous Co 3 O 4 materials with hierarchical structures were obtained, showing the signature of a bimodal pore-size distribution and larger BET specific surface area and pore volume. These hierarchical mesoporous Co 3 O 4 materials exhibit superior Li storage performance than the mesoporous Co 3 O 4 with mono-sized pores due to their smaller charge transfer impedances. Besides, 3D cubic mesoporous Co 3 O 4 is more beneficial for Li ion storage than 2D hexagonal mesoporous Co 3 O 4 . Reversible discharge specific capacities around 1141 mAh g −1 were obtained over the hierarchically porous Co 3 O 4 materials at a current density of 50 mA g −1 , which are comparable with or even higher than those reported in the literature. Hence, the as-prepared well-ordered mesoporous Co 3 O 4 with hierarchical structure could be the promising anode materials for high performance Li-ion batteries.

Methods
Synthesis of KIT-6 and SBA-15 silica. 3D cubic Ia3d KIT-6 mesoporous silica materials were prepared according to the procedure described by Ryoo and co-workers 34 . In a typical synthesis, 6 g of P123 was dissolved in 217 mL of distilled water with 10 mL of conc. HCl (37 wt%). 7.41 mL of n-butanol was added to the mixture under stirring at 35 °C. Then, this mixture was stirred for 1 h at 35 °C before 13.87 mL of TEOS was added. After stirring at 35 °C for another 24 h, the mixture was subsequently transferred into stainless-steel autoclaves, followed by the hydrothermally treated at 100 °C for 24 h. The resulting mixture was filtered without washing and dried at 80 °C. The organic template was removed by calcination at 550 °C for 6 h in air at a heating rate of 3 °C min −1 . The product was nominated as KIT-6-100 ("100" denotes the hydrothermal treating temperature of KIT-6). In another set of experiments, the hydrothermal treating temperature was varied from 40 °C to 130 °C.
2D hexagonal P6mm SBA-15 mesoporous silica material was synthesized according to the literature 35 with the hydrothermal treating temperature of 100 °C. The product was nominated as SBA-15-100.  Materials characterization. Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/max 2200 X-ray diffractometer using Ni-filtered Cu Kα radiation (λ = 0.15418 nm) operating at 40 kV and 40 mA. Transmission electron microscopy (TEM) images were measured on a JEOL JEM-2010 transmission electron microscope equipped with an Oxford energy-dispersive X-ray (EDX) spectrometer attachment operating at 200 kV. Nitrogen adsorption-desorption isotherms were measured on a Micromeritics ASAP 2020M analyzer at liquid nitrogen temperature (77 K). Prior to determination of the isotherm, the samples were degassed at 423 K in vacuum for 5 h. The Brunauer-Emmett-Teller (BET) specific surface area was calculated using the adsorption data in the relative pressure (P/P 0 ) range from 0.05 to 0.3, and the total pore-volume was determined from the amount adsorbed at P/P 0 = 0.98. The pore-size distribution curve was calculated based on the desorption branch of the isotherm using the Barrett-Joyner-Halenda (BJH) method. The pore diameter was defined as the position of the maximum in the pore-size distribution.
Electrochemical Test. Electrochemical performance of the powders was evaluated with two-electrode CR2032-type coin cells with a lithium foil counter electrode and an electrolyte consisting of a 1 M LiPF 6 solution in EC/DMC (1:1 by volume). Microporous polypropylene membrane (celgard 2400) was used as the separator. The working electrode was constructed from a paste consisting of 75% active powder, 15% conductive acetylene black and 10% PVDF binder in NMP solvent. The paste was cast onto Cu foil and finally dried at 100 °C under vacuum for 12 h before electrochemical evaluation. The loading weight of the active material on the electrode is about 2 mg. The cell assembly was operated in an argon-filled glove box (VAC AM-2) with oxygen and water contents less than 1 ppm. Cyclic voltammetry (CV) measurement of the electrode was performed between 3.0 and 0.01 V at a scan rate of 0.5 mV s −1 using an electrochemical workstation (CHI 604C). The galvanostatic charge and discharge test was carried out using a LAND CT2001A battery test system in the voltage window of 0.01-3.0 V at a current density of 50 mA g −1 . AC impedance of the cell was measured by a Frequency Response Analyzer (FRA) technique on an Autolab Electrochemical Workstation over the frequency range from 10 5 Hz to 0.01 Hz with the amplitude of 5 mV. All the electrochemical measurements were conducted at room temperature.