Nanoparticle Decorated Ultrathin Porous Nanosheets as Hierarchical Co3O4 Nanostructures for Lithium Ion Battery Anode Materials

We report a facile synthesis of a novel cobalt oxide (Co3O4) hierarchical nanostructure, in which crystalline core-amorphous shell Co3O4 nanoparticles with a bimodal size distribution are uniformly dispersed on ultrathin Co3O4 nanosheets. When tested as anode materials for lithium ion batteries, the as-prepared Co3O4 hierarchical electrodes delivered high lithium storage properties comparing to the other Co3O4 nanostructures, including a high reversible capacity of 1053.1 mAhg−1 after 50 cycles at a current density of 0.2 C (1 C = 890 mAg−1), good cycling stability and rate capability.

Rechargeable lithium-ion batteries (LIBs) are one of the great successes of modern materials electrochemistryused in electronics, mobile phones, and laptop computers 1,2 . High-performance LIBs with higher capacity, longer cycle life, and better rate capability have attracted considerable interests in the electric vehicle market and for implantable medical devices etc [1][2][3][4][5] . It is widely accepted that the overall performance of LIBs is highly dependent on the inherent electrochemical properties of the electrode materials 6,7 . Therefore, considerable attention has been paid to develop novel materials for both the cathodes and anodes of LIBs which are inexpensive, safe and environmentally benign. So far, various materials, such as graphitic/non-graphitic carbon [8][9][10] , transition-metal oxides 11,12 , nitrides 13,14 , phosphates 15 , lithium alloys [16][17][18][19][20] and their composites have been exploited as the anode materials of LIBs. Among them, Co 3 O 4 is a promising material for the LIB anode due to its high theoretical capacity (890 mAhg −1 , according to the electrochemical reaction Co 3 O 4 + 8Li + + 8e − ↔ 3Co + 4Li 2 O). The capacity is more than two times larger than that of graphite (372 mAhg −1 ), which is anticipated to meet the requirements of future energy storage systems 21 . Nevertheless, the main weakness of Co 3 O 4 anode materials for LIBs lies in the large volume expansion and severe particle aggregation associated with the Li + insertion and extraction process, resulting in the deterioration of the reversible capacity and poor cycling stability 21 . Various strategies have been attempted to overcome these limitations and improve the performance. Design and synthesis of electrode materials with proper composition, morphology (such as nanopowders, nanowires, nanorods and nanotubes), and microstructure on the nanoscale is a time-tested route to enhance the lithium storage properties [22][23][24][25] (see Co 3 O 4  anode nanostructures overview in Table S1). Li et al. 24 syntheized self-supported mesoporous Co 3 O 4 nanowire arrays directly grown on Ti foil, which showed high capacity, good cyclability and high rate capability. Nam and Belcher et al. 25 used viruses to synthesize and assemble Au modified Co 3 O 4 nanowires at room temperature, and demostrated the improved battery capacity when used as anodes for LIBs. Combining nanostructured electrode materials with electronically conductive agents, such as carbon nanofibers, carbon nanotubes, and graphene, is considered as another effective approach to improve the cycling stability and rate capability 26,27 . The conductive additives not only act as a "buffer zone" of volume variation induced by the cycling process but also a good

Results
The route to obtain Co 3 O 4 C@A NPs-NSs HNs was adapted from our previously described synthesis of mesoporous Co 3 O 4 nanosheets with some modifications (see the Supporting Information (SI) for details). Briefly, the cobalt hydroxide sheet precursors were obtained by reacting Co(NO 3 ) 2 ·6H 2 O, hexamethylenetetramine, and water at 95 °C for 8 h. The product was then annealed at 450 °C in air for 2 h to form Co 3 O 4 nanosheets that have previously been investigated 36 . In the last step, the annealed samples were soaked in NaBH 4 solution for an hour to yield C@A NPs-NSs HNs.
The morphology of the as prepared precursors shows sheet-like character (Fig. S1a, Supporting Information). EDX pattern confirms the composition of the precursor (Fig. S1a inset, SI). Powder X-ray diffraction (XRD) result indicates all the diffraction peaks can be indexed as Co(OH) 2 (JCPDS No. 74-1057) (Fig. S1b, SI). The thermal behavior of the Co(OH) 2 precursor is studied by TG analysis (Fig. S2, SI). From the TGA curve measured under air atmosphere, it can be clearly seen that the weight loss takes place in the temperature range 50-220 °C, and it is mainly attributed to the removal of physically adsorbed water and partial decomposition of the solid precursor into Co 3 O 4 nanosheets. After reaching 250 °C, the weight loss drops sharply to about 30%. Then the weight loss is gradual and finally it flattens at 350 °C. On the basis of these results the as-synthesized precursors were annealed at 450 °C. The structure and morphology characterizations indicate that the annealed products are phase pure Co 3  . The average diameters of the two kinds of nanoparticles are ~6 and ~45 nm, respectively. Although various 0D nanoparticles (metal, metal oxides/sulfide/nitrides) have been successfully deposited on 2D nanosheets (such as graphene, reduced graphene oxide, layered transition metal dichalcogenide nanosheets, atomically-thick nanosheets with non-layered structure, and so on) 27,[37][38][39] , to the best of our knowledge, this is the first report on the synthesis of 0D-2D hybrid nanostructures for a given material via a facile solution method.
The detailed structural investigations of Co 3 O 4 C@A NPs-NSs HNs are studied by TEM and HRTEM. Figure 3(a-d) are typical TEM images of Co 3 O 4 C@A NPs-NSs HNs with different magnifications; it can be seen that the hierarchical nanosheets are highly porous (Fig. S5a,b). The sheets are folded in some locations and the dark regions are the result of overlapping of sheets. Moreover, the porous nanosheets are decorated with high-density nanoparticles. A bimodal size distribution with mean diameters of ~5.5 nm and ~43 nm are obtained by analyzing over 200 nanoparticles for the sample (the inset in Fig. 3b). The results are in good agreement with the above FESEM observations. A selected area electron diffraction pattern (   The surface chemical composition and oxidation state of Co 3 O 4 C@A NPs-NSs HNs are determined by employing XPS analysis. The XPS spectrum of the HNs in the region of 0-1300 eV is shown in Fig. 4(a) and confirms the peaks of Co and O. The high resolution scan of Co 2p (Fig. 4b) exhibits two peaks located at 795.6 eV and 780.2 corresponding to the electronic states of Co 2p 1/2 and Co 2p 3/2 respectively. The presence of Co 3 O 4 can be further confirmed by the O 1s peak (Fig. 4c) located at 530.2 eV, which corresponds to the oxygen species forming oxide with cobalt elements.
Nitrogen adsorption-desorption isotherm is used to determine the specific surface area and the porous nature of the product. The N 2 adsorption-desorption isotherm of Co 3 O 4 C@A NPs-NSs HNs at 77K is presented in Fig. 4(d) with the inset displaying the corresponding Barret-Joyner-Halenda (BJH) pore size distribution. The isotherm shows a hysteresis loop at relative pressure range of 0.8-1.0 P/P 0 and the Bruauer-Emmett-Teller (BET) specific surface area is 51.9 m 2 g −1 , which is higher than that of Co 3 O 4 nanosheets 36 . The higher surface area of Co 3 O 4 C@A NPs-NSs HNs may be attributed to the mesoporous nature of the nanosheets and the void spaces between the nanoparticles. The BJH pore size distribution curves indicate that the average pore size is ~16 nm for large pores and ~2 nm for small pores (inset in Fig. 4d). The present porous Co 3 O 4 C@A NPs-NSs HNs are of importance in lithium-storage process, due to their capability of providing extra active sites for the storage of lithium ions and facilitating mass diffusion and ion transport, which are induced by the synergistic reactions of porous structures and the specially designed structure of the constituent nanoparticles.
We subsequently study the electrochemical properties of Co 3 O 4 C@A NPs-NSs HNs as an anode material for LIBs. Figure 5  cathodic peak at ~0.62 V which can be ascribed to the electrochemical reduction (lithiation) reaction of Co 3 O 4 with Li. In the following anodic scan the anodic peak at ~2.2 V is ascribed to the oxidation (delithiation) reaction of Co 3 O 4 C@A NPs-NSs HNs. In the second cycle, the main reduction and oxidation peaks are shifted to ~0.78 V and ~2.16 V respectively, and the intensity of reduction peaks decreases due to the formation of irreversible solid electrolyte interface (SEI) film and the irreversible reduction reaction 40,41 . The peaks intensity and the integral areas of the third cycle are very close to that of the second cycle. These results show a gradual improvement in the electrochemical reversibility of Co 3 O 4 C@A NPs-NSs HNs after the first cycle. Figure 5b shows the representative galvanostatic charge-discharge voltage profiles of Co 3 O 4 C@A NPs-NSs HNs at a current density of 0.2 C (1 C = 890 mAhg −1 ) in the voltage window of 0.01-3 V (versus Li + /Li) at room temperature. Similar to the previous report 40,41 , in the first discharge curve, the potential value quickly falls to the plateau (~1.00 V), and the extended plateau with a ~900 mAh/g capacity may likely be ascribed to the conversion from Co 3 O 4 to Co, and then gradually declines to the cutoff voltage (0.01 V), which could be associated with the formation of a polymer/gel-like film on the surface of Co 3 O 4 particles 40,41 . The electrode delivers first-cycle discharge and charge capacities of 1349.4 and 1025.6 mAhg −1 , respectively, yielding an irreversible capacity loss of 24%. Such initial irreversible capacity loss mainly originates from the formation of SEI layer due to the irreversible degradation of the electrolyte and other irreversible side reactions [22][23][24][25][26]35,40 . The following two (2 nd and 3 rd ) discharge/charge curves tend to be stable and exhibit similar electrochemical behavior. The discharge and charge capacities are 1165.3 and 1111.8 mAhg −1 for the second cycle, and 1150.2 and 1112.2 mAhg −1 for the third cycle, corresponding to the Coulombic efficiency of 95.4% and 96.7%, respectively. We notice that all of the capacities as mentioned above are higher than the theoretical total capacity of Co 3 O 4 (890 mAhg −1 ), which is probably caused by the reversible formation/dissolution of the polymer/gel-like film contributing to an additional reversible capacity besides the electrochemical conversion reaction between cobalt oxide and Co 40,41 . Figure 5(c) shows the discharge-charge cycling performance of Co 3 O 4 C@A NPs-NSs HNs evaluated between 0.01 and 3V (versus Li + / Li) at a current density of 0.2 C at room temperature. It is found that the capacities of the following cycles from the fourth cycle increase slowly and gradually. The maximum discharge capacities could reach up to 1262 mAhg −1 . The possible reason is that the diffusion of Li-ion is activated and stabilized gradually during cycling process 42   To further evaluate the rate capability, the Co 3 O 4 C@A NPs-NSs HNs electrode is cycled at various current densities between 0.2 C and 5 C as shown in Fig. 5(d). The charge/discharge rates are programmably modified from 0.2 C to 1 C, 2 C, 5 C and then back to 0.2 C for 10 cycles. It can be found that the discharge and charge capacities remain stable and decrease regularly with an increased current rate. After every 10 cycles at a specific current rate, the reversible capacity at 0.2, 1, 2, and 5 C are about 1151.9, 804, 216.9 and 22.2 mAhg −1 , respectively. When the current density is decreased from 5 to 1 C, the reversible capacity can be recovered to 1186.3 mAhg −1 . These results demonstrate that the Co 3 O 4 C@A NPs-NSs HNs electrode has good electrochemical reversibility.

Discussion
The lithium-storage properties, including the capacity, cycling performance especially the rate capacity of the Co 3 O 4 C@A NPs-NSs HNs electrode are superior to that of most nanostructured Co 3 O 4 materials, such as porous Co 3 O 4 NWs arrays, Co 3 O 4 nanowires, flower-like porous Co 3 O 4 spheres, porous Co 3 O 4 needles, and so on (see Table S1). The good lithium storage properties of Co 3 O 4 C@A NPs-NSs HNs might be attributed to the rationally designed hierarchical nanostructures. Firstly, the porous nanosheets and the amorphous shell of the Co 3 O 4 nanoparticles can accommodate the local volume change upon charge/discharge cycling and is likely to alleviate the problem of pulverization and aggregation of the electrode material, thus leading to improved cycling stability [43][44][45][46][47] . Furthermore, the hierarchical architecture assembled with 0D nanoparticles and 2D nanosheets is favorable for preventing the aggregation of the constituted nanobuilding blocks, also improving the cycling performance 35,48 . Secondly, the hierarchical structures with high density Co 3 O 4 decorated nanosheets provide sufficient electrode-electrolyte contact area for the storage of lithium ions, which is beneficial for enhancing the specific capacity. Moreover, the amorphous shell could offer additional reaction sites on the surface, which is also responsible for the high specific capacity of the Co 3 O 4 C@A NPs-NSs HNs electrodes 43,45,47 . Finally, the crystalline Co 3 O 4 cores and nanosheets have the functions of providing stable mechanical support and an efficient electrical conducting pathway, while the amorphous Co 3 O 4 shells could give reliable continuous pathways for Li + during the course of continuous charge/discharge processes, enhancing the kinetics and structural stability for lithium storage 47 . As a result of the above-mentioned advantages, the prepared Co 3 O 4 C@A NPs-NSs HNs electrode expectedly manifests enhanced lithium storage properties.
In conclusion, we report the synthesis of unique Co 3 O 4 C@A NPs-NSs HNs, in which crystalline@amorphous core/shell Co 3 O 4 nanoparticles with a bimodal size distribution uniformly disperse on ultrathin Co 3 O 4 nanosheets. When used as the anode materials of LIBs, the as-prepared Co 3 O 4 C@A NPs-NSs HNs electrodes delivered high lithium storage properties, including a high reversible capacity of 1053.1 mAhg −1 after 50 cycles at a current density of 0.2 C, good cycling stability and rate capability. It is believed that the excellent electrochemical performance can be attributed to the uniquely designed hierarchical nanostructures. The present facile synthesis route can be applied to other metal oxides with desirable nanostructures and functions.

Methods
Materials Synthesis. All of the reagents are analytical grade and used as received. Firstly, 1.45 g of Co(NO 3 ) 2 ·6H 2 O and 1.4 g of HMT (hexamethylenetetramine, C 6 H 12 N 4 ) were dissolved in 30 ml of water under stirring for 30 min. The mixture was then transferred into a Teflon-linked stainless steel autoclave (50 ml capacity). The autoclave was sealed and maintained at 95 °C for 8 h. After cooling down to room temperature spontaneously, the precipitate is rinsed with distilled water and ethanol, and dried at 60 °C under vacuum for 2 h. Finally,the product was annealed at 450 °C in air for 2 h. The annealed samples were then soaked in 1M NaBH 4 solution (20 ml distilled water). The sample was collected after an hour and washed with distilled water three times. Finally, the product was collected after centrifugation and dried in an oven at 70 °C for 10 h.
Characterization of Materials. The phases of the unreduced and reduced products were characterized by X-ray diffraction (XRD). The morphology and structure of the products were obtained by field emission scanning electron microscopy (FESEM, Hitachi S5500), transmission electron microscopy (TEM, FEI Tecnai G 2 20, 200 kV), high resolution transmission electron microscopy (HRTEM, FEI Titan 80-300, 300 kV), and X-ray photoelectron spectroscopy (XPS, Escalab 250, Al Kα ). The elemental mapping was done by using energy dispersive X-ray spectroscopy (EDS). The surface area of the products was measured by the Bruauer-Emmett-Teller (BET) method using nitrogen adsorption-desorption isotherm. Pore size distribution plots were obtained by the Barret-Joyner-Halenda (BJH) method. Thermogravimetric (TG) analysis was carried out on a TGA 2050 thermogravimeteric analyzer under an air atmosphere at the temperature range of 25-500 °C with a heating rate of 10 °C min −1 .

Electrochemical Measurements.
To measure the electrochemical performance, the electrodes were constructed by mixing the active materials, conductive carbon black and carboxymethyl cellulose, in a weight ratio of 80:10:10. The mixture was prepared as slurry and spread onto copper foil. The electrode was dried under vacuum at 120 °C for 5 h to remove the solvent before pressing. Then the electrodes were cut into disks (12 mm in diameter) and dried at 100 o C for 24 h in vacuum. The cells were assembled inside an Ar-filled glove box by using a lithium metal foil as the counter electrode and the reference electrode and microporous polypropylene as the separator. The electrolyte used was 1M LiPF 6 dissolved in a mixture of ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC) with a volume ratio of EC/PC/DEC = 3:1:1. The assembled cells were allowed to soak overnight, and then the electrochemical tests on a LAND battery testing unit were performed. Galvanostatic charging and discharging of the assembled cells were performed at different current rates between voltage limits of 0.01 and 3V (vs. Li + /Li) at room temperature. The cyclic voltammogram (CV) was performed at a scan rate of 0.5 mVs −1 in the range of 0.01−3.00 V (vs. Li + /Li).