Growth of Ultrathin ZnCo2O4 Nanosheets on Reduced Graphene Oxide with Enhanced Lithium Storage Properties

The growth of ultrathin ZnCo2O4 nanosheets on reduced graphene oxide (denoted as rGO/ZnCo2O4) via a facile low‐temperature solution method combined with a subsequent annealing treatment is reported. With the assistance of citrate, interconnected ZnCo2O4 nanosheets can assemble into hierarchically porous overlays on both sides of rGO sheets. Such a hybrid nanostructure would effectively faciliate the charge transport and accommodate volume variation upon prolonged charge/discharge cycling for reversible lithium storage. As a result, the rGO/ZnCo2O4 nanocomposite manifests a very stable high reversible capacity of around 960 mAh g−1 over 100 cycles at a low current density of 90 mA g−1 and excellent rate capability.

benignity. [ 16,18,19 ] Importantly, ZnCo 2 O 4 can store Li + through not only the conversion reaction, but also the alloying/dealloying reaction between Zn and Li, which results in a high theoretical capacity of ca. 900 mAh g −1 . [ 13 ] Very recently, ZnCo 2 O 4 anode materials with various morphologies such as nanoparticles, porous nano/ microspheres and nanotubes/nanowires/ nanorods have been synthesized and applied as anode materials for LIBs with high capacity. [ 4,14,[16][17][18][19][20][21] Nevertheless, the intrinsic poor electric conductivity and low cycling stability due to drastic volume change during lithium insertion/extraction process still limit the practical application of ZnCo 2 O 4 -based electrodes. [ 17 ] To circumvent these problems, one effective strategy is to employ a suitable fl exible matrix to accommodate the volume variation and improve the electric conductivity at the same time. [ 22 ] In this regard, graphene or reduced graphene oxide (rGO), has been widely investigated and proven as an effective conducting support to host TMOs in high-power LIBs, because of its outstanding characteristics, including high electrical conductivity, excellent mechanical fl exibility, large specifi c surface area, and high chemical stability. [23][24][25][26][27][28][29] Generally, rGO in the nanocomposites could not only increase the electric conductivity, but also provide an elastic buffering support to withstand the huge volume change and drastic structural re-organization of TMOs, thus leading to improved cycling stability. [ 25,26 ] Although tremendous efforts have been devoted to coupling graphene with different TMOs, hybrid nanostructures of graphene supported ZnCo 2 O 4 nanosheets as electrode materials for LIBs have not been realized so far.
Herein, we develop a facile two-step strategy to design and fabricate a unique hierarchical hybrid structure of rGO supported ZnCo 2 O 4 nanosheets (denoted as rGO/ZnCo 2 O 4 ) as an advanced anode material for high performance LIBs. With the assistance of trisodium citrate, ultrathin ZnCo 2 O 4 nanosheets can assemble into a hierarchically porous fi lm that fully covers both sides of rGO sheets. With the structural and compositional advantages, the as-synthesized rGO/ZnCo 2 O 4 nanocomposite is expected to manifest enhanced lithium storage properties.

Materials Synthesis
Graphene oxide (GO) was fi rst synthesized based on a modifi ed Hummer's method. [ 30 ] In a typical synthesis of reduced GO

Introduction
Lithium-ion batteries (LIBs) have gained commercial success as the leading power source for portable electronics, and have shown great promise in upcoming large-scale applications. [1][2][3][4] The evergrowing market demands for LIBs have stimulated numerous research efforts aiming at the exploration of novel electrode materials with higher capacity and long-term cycling stability. [ 5,6 ] Transi tion metal oxides (TMOs), especially cobalt-based oxides with a spinel structure, have been intensively investigated as potential alternatives to graphite-based anode materials for their higher theoretical capacities. [ 7,8 ] However, cobalt-based oxides are limited by their high cost and toxicity. [ 9,10 ] Thus, extensive research efforts are now made to fabricate novel ternary cobalt oxides by partially substituting Co with less expensive and eco-friendly metals. Moreover, ternary metal oxides usually own many unique properties originated from the co-existence of two types of different cations in a single crystal structure. [11][12][13][14][15][16][17] Amongst a variety of candidates, ternary ZnCo 2 O 4 has been considered attractive in view of its enhanced cycling stability and good environ mental After ultrasonication for another 5 min, the mixed solution was refl uxed in an oil bath at 90 °C for 6 h. After being collected by centrifugation and rinsed with DI water and ethanol for several times, the obtained Zn-Co precursor grown on rGO was dried overnight at 80 °C. Finally, the product was annealed at 400 °C for 3 h in N 2 atmosphere with a slow heating rate of 1 °C min −1 to generate well-defi ned rGO-supported ZnCo 2 O 4 nanosheets.

Materials Characterization
X-ray diffraction (XRD) patterns were obtained on a Bruker D2 Phaser X-Ray Diffractometer with Ni fi ltered Cu Kα radiation (λ = 1.5406 Å) at a voltage of 30 kV and a current of 10 mA. Field-emission scanning electron microscope (FESEM) images were obtained by a JEOL JSM-6700F microscope operated at 5 kV. Transmission electron microscope (TEM) images were recorded by JEOL JEM-2010 and JEOL JEM-2100F microscopes. Thermogravimetric analysis (TGA) was carried out under air fl ow of 200 mL min −1 with a temperature ramp of 10 °C min −1 . Nitrogen sorption measurement was acquired on Autosorb 6B at -196 °C.

Electrochemical Measurements
The electrochemical tests were conducted in two-electrode Swagelok cells. The working electrodes consisted of 70 wt% of active materials, 20 wt% of conductive carbon black (Super-P-Li), and 10 wt% of polymer binder (polyvinylidene fl uoride, PVDF). The electrolyte is 1 M LiPF 6 in a mixture of ethyl ene carbonate and diethyl carbonate (1:1 by weight). Lithium discs were used as both the counter electrode and reference electrode. Cell assembly was carried out in an Ar-fi lled glovebox (Inno vative Technology Inc.) with moisture and oxygen concentrations below 1.0 ppm. The galvanostatic charge-discharge measurements were performed within a voltage window of 0.01-3 V on a NEWARE battery tester.

Results and Discussion
In the present synthesis, two steps are involved to synthesize hierarchical ZnCo 2 O 4 -rGO, as illustrated in Figure 1 . Specifically, GO sheets are fi rst dispersed into an aqueous solution containing Zn(NO 3 ) 2 , Co(NO 3 ) 2 , hexamethylenetetramine (HMT) and trisodium citrate (TSC). During the refl uxing process, decomposition of HMT results in the formation of Zn-Co precursor. Due to the strong coordination effect between the function groups of GO sheets and metal ions, the Zn-Co precursor selectively grows on the surface of GO sheets. Besides, it's worth mentioning that the hydrolysis of TSC can further promote formation of Zn-Co precursor into unique ultrathin nanosheets standing upright on both sides of the GO sheets (denoted as rGO/Zn-Co precursor). In the growth process, GO is expected to be partially reduced by the reducing species generated from HMT and TSC. In the second step, the Zn-Co precursor can be easily transformed to crystalline ZnCo 2 O 4 with well-retained nanosheets morphology via a facile thermal annealing treatment in N 2 at 400 °C. As a result, the novel rGO/ ZnCo 2 O 4 hierarchical hybrid structure can be obtained.
The obtained nanocomposite is fi rst characterized by powder X-ray diffraction (XRD) to determine their crystallographic structures. The as-prepared Zn-Co precursor is nearly amorphous ( Figure S1, see Supporting Information). However, after annealing at 400 °C in N 2 for 3 h, all of the identifi ed diffraction peaks in the XRD pattern of the annealed product confi rm the formation of the spinel ZnCo 2 O 4 phase (JCPDS card no. 23-1390) without noticeable signals of possible crystalline impurities ( Figure 2 A). [ 13,17 ] The morphology and structure of the pristine GO sheets and as-prepared rGO/ZnCo 2 O 4 hybrid are further examined by fi eld-emission scanning electron microscopy (FESEM).  Figure S2, see Supporting Information), suggesting that the GO substrate is able to prevent the severe aggregation of ZnCo 2 O 4 nanosheets effectively. The highly porous feature of the composite is characterized by N 2 adsorption-desorption measurement ( Figure S3, see Supporting Information), which reveals a high Brunauer-Emmett-Teller (BET) specifi c surface area of about 186.6 m 2 g −1 . In addition, thermogravimetric analysis (TGA) shows that the weight fraction of the rGO support is about 13.7 wt% in the fi nal nanocomposite ( Figure S4, see Supporting Information). Such a porous architecture with conductive graphene support holds great promise in offering Due to the low contrast of graphene and the presence of large amount of ZnCo 2 O 4 nanosheets, the graphene support cannot be directly observed under TEM. In addition, the porous structure of these ultrathin ZnCo 2 O 4 nanosheets can be clearly observed in a highmagnifi cation TEM image ( Figure S5, see Supporting Information), which agrees well with the above BET analysis. The formation of porous structure is mainly due to the gradual decomposition of the precursor (hydroxide and carbonate) during the annealing process. [ 24 ] Consistent with XRD analysis, a set of distinct lattice fringes with a spacing of 0.24 nm can be observed in the high-resolution TEM image of a typical ZnCo 2 O 4 nanosheet (Figure 3 C), which corresponds to the (311) crystal planes of the spinel ZnCo 2 O 4 phase. [ 31 ] Furthermore, the selected area electron diffraction (SAED) pattern (Figure 3 D) indicates a polycrystalline nature of the nanosheets and the diffraction rings can be readily assigned to the crystal planes of the spinel ZnCo 2 O 4 phase.
It is worth mentioning that the presence of TSC in the reaction plays a crucial role in the formation of the hierarchical hybrid structure of ultrathin nanosheet subunits on the rGO substrate. [ 28,32 ] Without the addition of TSC, only some irregular ZnCo 2 O 4 nanoparticles or nanospheres can be found on the surface of rGO substrate ( Figure 4 A). When a small amount of    (Figure 4 B), which indicates that heterogeneous nucleation of Zn-Co precursor nanosheets on GO support has been facilitated by the functional groups of TSC. Increasing the amount of TSC to 0.05 mmol, these irregular nanosheets evolve into slender nanosheets, which start to stand on the surface of rGO substrate (Figure 4 C). Further increasing the amount of TSC leads to the formation of large and up-standing nanosheets on GO sheets with high uniformity. In particular, when the amount of TSC is 0.15 mmol, the as-prepared hybrid structure manifests the optimal morphology, which consists of densely standing and interconnected nanosheets (Figure 4 E). Nevertheless, upon increasing the amount of TSC to 0.25 mmol, the packing of nanosheets becomes denser and some agglomeration starts to appear on the surface of rGO sheets (Figure 4 F). Clearly, the morphology of the hierarchical rGO/ZnCo 2 O 4 hybrid structure can be tuned by simply controlling the amount of TSC in the reaction solution through the possible coordination effect between metal ions and functional groups of TSC. Meanwhile, HMT simply serves as the alkaline source to trigger the formation of Zn-Co precursor with sphere-like nanostructures fi rmly anchoring onto rGO sheets. [ 33 ] Without TSC, HMT can only lead to the precipitation of irregular particles of Zn-Co precursor on rGO sheets ( Figure S6, see Supporting Information). We next evaluate electrochemical properties of the rGO/ ZnCo 2 O 4 nanocomposite as an anode material for LIBs. can be clearly identifi ed from the CVs, indicating the similar electrochemical reaction mechanism. [ 13,18 ] In the fi rst cycle, the irreversible cathodic peak located at around 0.50 V can be attributed to the reduction of ZnCo 2 O 4 to metallic Zn and Co. The signifi cant decrease in the peak intensity in the subsequent scans indicates the existence of some irreversible processes during the fi rst cycle, whereas the shift of peak position to higher potentials in the following cycles might be related to some activation process for the Li + insertion in the fi rst cycle. Meanwhile, two broad anodic peaks centered at about 1.68 and 2.29 V in the following anodic scan can be ascribed to the oxidation of metallic Zn and Co to ZnO and CoO x, respectively. Thus, on the basis of above CV analysis and previous reported lithium storage mechanisms of ZnO, CoO and Co 3 O 4 , the lithium insertion/extraction reactions for our rGO/ZnCo 2 O 4 electrode might be described as follows: [ 19,34 ]    respectively. The corresponding irreversible loss of about 36% during the fi rst charge is commonly attributed to irreversible side reactions on the surface of electrodes, such as formation of solid-electrolyte interface (SEI) layer, and possible incomplete restoration of metallic Zn and Co into the original oxides. [ 13,16,[34][35][36] Nevertheless, the voltage profi les approximately overlap except for the initial discharge, indicating good reversibility of the electrochemical reactions of the material for reversible lithium storage. To evaluate the cycling stability, the rGO/ZnCo 2 O 4 electrode is charged and discharged at a current density of 90 mA g −1 , as depicted in Figure 5 C. As expected, the rGO/ZnCo 2 O 4 nanocomposite shows good capacity retention from the second cycle onwards and eventually delivers a reversible discharge capacity as high as 960.8 mAh g −1 in the 100 th cycle, corresponding to 89.7% of the second-cycle discharge capacity. As a comparison, without the rGO sheets support, the electrode of fl ower-like ZnCo 2 O 4 microspheres shows poor cycling stability ( Figure S7, see Supporting Information). After 100 cycles at the same current density, its discharge capacity decreases sharply to 421.6 mAh g −1 , corresponding to about 43% of the second-cycle discharge capacity. Owing to the unique structure, the rGO/ZnCo 2 O 4 nanocomposite manifests excellent capacity retention at continuously varying current densities ranging from 90 to 900 mA g −1 as shown in Figure 5 D. The specifi c capacity of rGO/ZnCo 2 O 4 decreases steadily as the current density increases, but still retains high values. For example, at a high current density of 900 mA g −1 , the rGO/ ZnCo 2 O 4 electrode is still able to deliver a stable discharge capacity of about 593.2 mAh g −1 . Remarkably, the capacity could resume to a high value of 963.7 mAh g −1 when the current density is reduced back to 90 mA g −1 , indicating good reversibility of the electrode material. Moreover, because of its excellent robustness, the morphology and structure of the rGO/ZnCo 2 O 4 nanocomposite are perfectly retained after ten charge-discharge cycles at 90 mA g −1 ( Figure S8, see Supporting Information).
Clearly, the rationally designed nanostructure and composition of rGO/ZnCo 2 O 4 are benefi cial for the enhanced electrochemical performance. Specifi cally, the hierarchically porous structure assembled by ultrathin ZnCo 2 O 4 nanosheets on the fl exible rGO substrate provides suffi cient electrode-electrolyte contact area for high Li + ion fl ux across the interface and at the same time shortens Li + ion diffusion distance, thus greatly facilitating the electrochemical processes especially at high current densities. [ 37 ] On the other hand, open and porous frameworks and the fl exible rGO substrate could better improve the electrode stability by effectively mitigating the internal mechanical stress during repeated charging-discharging processes, as well as preventing the nanostructures from agglomeration. [ 38 ] Finally, the rGO substrate with relatively good electrical conductivity might improve the reaction kinetics towards fast lithium insertion/extraction. [ 9,39 ]

Conclusion
In summary, we have developed a simple strategy to grow ultrathin ZnCo 2 O 4 nanosheets onto reduced graphene oxide (rGO) sheets for enhanced lithium storage properties. The synthesis involves growth of precursor nanosheets on rGO surface and a subsequent thermal treatment. The morphology of this novel hybrid structure can be controlled by the added amount of trisodium citrate (TSC) in the reaction solution. The hierarchical rGO/ZnCo 2 O 4 nanocomposite demonstrates high reversible lithium storage capacity of 960.8 mAh g −1 over 100 cycles at the current density of 90 mA g −1 , and remarkable capacity retention at increased current densities as an advanced anode material for LIBs. Therefore, the present work offers a simple and effective approach for the development of high-performance electrode materials for advanced lithium ion batteries.