Combining Zn0.76Co0.24S with S-doped graphene as high-performance anode materials for lithium- and sodium-ion batteries

An easy and facile hydrothermal method is presented to synthesize hybrid materials of hollow mesoporous Zn0.76Co0.24S nanospheres anchored on reduced graphene oxide (rGO) sheets (Zn0.76Co0.24S@N/S-rGO), in which the obtained Zn0.76Co0.24S nanospheres are composed of numerous nanoparticles. Being evaluated as anode materials for lithium-ion batteries (LIBs) and sodiumion batteries (SIBs), the Zn0.76Co0.24S@N/S-rGO composites exhibited a high reversible capacity of 804 and 605mAhg at the current density of 1 A g after 500 cycles for LIBs and SIBs, respectively. The excellent electrochemical performance of Zn0.76Co0.24S@N/S-rGO composites originates from the synergistic effect between hollow Zn0.76Co0.24S nanospheres and reduction graphene, as well as the void spaces between the neighbouring nanoparticles of Zn0.76Co0.24S providing large contact areas with electrolyte and buffer zone to accommodate the volume variation during the cycling process.


Introduction
The exhaustion of non-renewable energy sources has brought about a series of environmental problems, which promote the development of innovative energy storage technologies to harvest the clean and sustainable energy [1]. Among various available energy storage technologies, rechargeable batteries play a significant role as power sources which have been widely applied in electric vehicles and various portable electronic devices [2]. Owing to their long lifespan, high power, and energy density, lithium-ion batteries (LIBs) have dominated the world's batteries market for many years [3]. Recently, due to the low cost and ubiquitous sodium resource, sodium-ion batteries (SIBs) have also attracted widespread attention as a promising alternative to LIBs [4]. Due to the high energy density, morphology-controlled, high theoretical capacities (500-1,000 mA h g −1 ), transition metal sulphides have been widely investigated and used in secondly energy storages [5]. However, currently neither of LIBs and SIBs could fully meet the ever-increasing demand for energy storage systems with high energy density [6]. Therefore, it is critical to develop advanced electrode materials with excellent property both for LIBs and SIBs [7].
One major challenge for both LIBs and SIBs is to develop suitable anode materials with high capacity, low redox potential as well as long cycle life [8]. The large volume change associated with sodiation/desodiation processes leads to the agglomeration of electrode particles along with electrical disconnection from current collectors, resulting in poor cycling stability [9]. As a type of promising anode materials, transitional metal chalcogenides have been attracting significant interests because of their superior conductivity and high theoretical capacity [10]. Nevertheless, there are still several problems that need to be solved before their application in practice, including poor cycle life and low rate performance, which are mainly caused by the serious volume expansion during charge/discharge [11].
Extensive efforts have been conducted to explore new strategies for the design and synthesis of transitional metal chalcogenide anodes with micro/nanostructure [12]. Hollow-structured materials have drawn great attention and proven to be efficient way to accommodate the volume fluctuation of transitional metal chalcogenides during cycling [13]. Such unique structures can not only provide large electrolyte/electrode contact area for rapid ion diffusion and reaction, but also effectively buffer the dramatic volumetric changes during electrochemical reactions. In addition, to improve the electronic conductivity of the metal chalcogenides and enhance the rate performance of the electrodes, electric-conductive, carbon matrix is commonly adopted to fabricate transitional-metal-chalcogenide/C composites [14]. Among numerous carbonaceous materials, graphene has been widely used as an ideal electrochemical matrix, due to its large surface area and outstanding electronic and electrochemical properties [15]. Therefore, the utilization of hollow framework in combination with graphene as conductive carbon material might be a synergistic strategy to address the aforementioned problems.
Herein, we demonstrated a facile and efficient hydrothermal synthesis method to prepare hollow-structured Zn 0.76 Co 0.24 S nanospheres anchored on rGO as anodes materials of LIBs and SIBs. The obtained Zn 0.76 Co 0.24 S@N/ S-rGO composite delivers high discharge capacities of 1,265 and 672 mA h g −1 at a current density of 0.1 A g −1 for LIBs and SIBs, respectively. The remarkable electrochemical performances of Zn 0.76 Co 0.24 S@N/S-rGO are mainly attributed to the following advantages. First, the mesoporous hollowstructured Zn 0.76 Co 0.24 S can provide large specific contact area for electrolyte/electrode contact. Second, the superior electrical conductivity of graphene leads to prominent electronic/ionic diffusion and enhanced structure stability. Third, the in situ sulfur doping can not only increase the capacity, but also improve the electrochemical properties of the carbon materials [16]. In this work, Zn 0.76 Co 0.24 S nanospheres were uniformly distributed and anchored on N/S-doped reduced graphene oxide (rGO) network through an effective solvothermal method (denoted as Zn 0.76 Co 0.24 S@N/S-rGO). As illustrated in Scheme 1, GO was dispersed in deionized water and sonicated for half an hour. Then, ZnNO 3 ·6H 2 O, CoNO 3 ·6H 2 O and L-cysteine were dissolved in GO solution and transferred to autoclave and heated at 180°C for 6 h. In the subsequent calcination process, the sulphur atoms derived from the L-cysteine were in situ doped in rGO network. The well-dispersed hollow Zn 0.76 Co 0.24 S nanospheres were tightly anchored on the GO network.

Material characterization
The crystal structure and composition information of all samples were analysed with Powder X-ray diffraction (XRD, Bruker D8 Advance X-ray diffractometer) with Cu Kα. The morphology of the obtained samples was characterized by field-scanning electron microscope (FESEM, JEOL 7500F, Japan) and transmission electron microscope (TEM, JEOL JEM-2100F, Japan). Nitrogen sorption isotherms were tested by V-Sorb X800 and X-ray photoelectron spectroscopy (XPS, VG Scientific) was tested to evaluate the valence of Zn, Co S and C.

Electrochemical measurements
The electrochemical properties of Zn 0.76 Co 0.24 S@N/S-rGO and Zn 0.76 Co 0.24 S were investigated in CR2032 half cells at ambient temperature. The obtained active materials of Zn 0.76 Co 0.24 S @N/S-rGO (70 wt%), acetylene black (20 wt%), and binder sodium alginate (SA) (10 wt%) were dispersed in DI water to form a homogeneous slurry. Then the obtained slurry was pasted on a copper foil, followed by vacuum drying at 110°C for 12 h. The copper discs with a diameter of 14 mm were punched out of copper foil and used as working electrode. The mass loading of active materials on cooper film was found to be about 1.5 mg cm −2 . For lithium batteries, the CR2032 coin cells were assembled in an argon-filled glovebox (UniLab, MBRAUN, Germany) with both moisture and oxygen levels below 0.1 ppm. LiPF 6 solution (1 M in mixed EC/DMC/EMC, in a volume ratio of 1:1:1) and a pure lithium foils were used as electrolyte and the counter electrode, respectively. For sodium batteries, 1.25 M NaPF 6 dissolved in ethyl methyl carbonate (EMC) were used as electrolyte and sodium foil was used as the counter electrolyte. Cyclic voltammetry (CV) was conducted in a CHI 660D electrochemical workstation within a voltage of 0.01-3.0 V at 0.1 mV s −1 . Electrochemical impedance (EIS, 100 kHz to 0.01 Hz) was performed in the same electrochemical workstation.

Results and discussions
The morphologies of Zn 0.76 Co 0.24 S nanospheres and Zn 0.76 Co 0.24 S@N/S-rGO composite were investigated via scanning electron microscope (SEM) and transmission electron microscope (TEM). The size of Zn 0.76 Co 0.24 S nanospheres ( Figure S1) is obviously larger than that of Zn 0.76 Co 0.24 S@N/S-rGO, which implies that the introduction of graphene could limit the size of nanospheres. Compared with pure Zn 0.76 Co 0.24 S nanospheres, the SEM images of Zn 0.76 Co 0.24 S@N/S-rGO show that the uniform nanospheres with an average size of 200 nm are anchored on RGO sheet (Figure 1a and b). Furthermore, the interaction of functional groups on rGO and Zn 0.76 Co 0.24 S nanospheres on rGO can effectively inhibit the aggregation and further growth of Zn 0.76 Co 0.24 S nanospheres demonstrated by the TEM images ( Figure 1 and Figure S1).  Figure S2) [18]. The energy-dispersive X-ray spectroscopy of Zn 0.76 Co 0.24 S@N/S-rGO shown in Figure S3 indicates that the element of Zn, Co, and S are present in the sample, which confirms that the composite was Zn 0.76 Co 0.24 S. Meanwhile, the EDX mapping images of Zn, Co, S, and C elements distinctly demonstrate the uniform distribution of the elements in the hollow nanospheres on GO nanosheet ( Figure S4). The Raman spectra of Zn 0.76 Co 0.24 S@N/S-rGO and GO samples are given in Figure 2b to better understand the surface layer of Zn 0.76 Co 0.24 S@N/S-rGO. The broad band located around 1,590 cm −1 corresponds to the E 2 g symmetric vibrational mode of graphite-type sp 2 carbons. The broad band around 1,350 cm −1 is an indicator of structural disorder, which is characterized by bond angle distribution and linked with sp 3 carbons [19]. The Raman spectra further confirm the residual carbon in the Zn 0.76 Co 0.24 S nanospheres, which is generated from the carbonization of organic molecules in the annealing process.
The Brunauer-Emmett-Teller (BET) analysis was carried out to evaluate the structural feature and porosity of Zn 0.76 Co 0.24 S@N/S-rGO and Zn 0.76 Co 0.24 S. The nitrogen adsorption-desorption isotherms and pore-size distribution curves of Zn 0.76 Co 0.24 S@N/S-rGO and Zn 0.76 Co 0.24 S are shown in Figure 3a and b, respectively. The BETspecific surface area was measured to be 40.9 m 2 g −1 for Zn 0.76 Co 0.24 S@N/S-rGO and 25.2 m 2 g −1 for Zn 0.76 Co 0.24 S. The nitrogen adsorption-desorption isotherm of Zn 0.76 Co 0.24 S@N/ S-rGO was found to coincide with type IV BET classification [20]. As the pore-size distribution curve shows, the pores of Zn 0.76 Co 0.24 S@N/S-rGO were mostly mesopores within 10 nm so that it could provide short and easy diffusion pathways for electrolyte ions to improve their electrochemical performance [21]. On the contrary, there were hardly any mesopores in Zn 0.76 Co 0.24 S. X-ray photoemission spectroscopy (XPS) measurements were carried out to further study the chemical composition and valence states of Zn 0.76 Co 0.24 S@N/S-rGO. XPS survey spectra of the Zn 0.76 Co 0.24 S@N/S-rGO identify the existence of Zn, Co, S, and C elements without any impurities (Figure 4a). The spectrum shows two peaks at 1,022.6 and 1,045.6 eV which can be assigned to Zn 2p 3/2 and Zn 2p 1/2 , respectively, indicating the existence of Zn 2+ (Figure 4b) [22]. The biding energy at 780.6 and 796.5 eV can be assigned to the 2p 3/2 and 2p 1/2 of Co 2+ , demonstrating the coexistence of Co 3+ and Co 2+ in the sample [23]. Two peaks can be discovered from the S 2p  spectrum in Figure 4d. The peak at 162.5 eV is a typical metal-sulfur bond in metal sulfides [24], while another peak at 169.0 eV is ascribed to sulfur oxides [25]. The C 1s HR-XPS spectrum shows four peaks located at 286.4, 285.4, 284.82, and 284.78 eV, which ascribed to the C═O, C-O, C-S, and C-C bonds, respectively [26]. Besides, after the hydrothermal reaction, rGO is not completely reduced because of the existence of O═C-O and C-O bonds. Due to the existence of C-S-C bond, it can act as the conjunction between the rGO and Zn 0.76 Co 0.24 S, resulting in the formation of a stable Zn 0.76 Co 0.24 S@N/S-rGO structure [27]. Moreover, the doped S atom changes the local charge distribution, which will enhance the local electron transfer, and thus, increase the electronic conductivity of the composite [28]. Figure 3f depicts the HR-XPS spectrum of N 1s of Zn 0.76 Co 0.24 S@N/S-rGO with three peaks: pyrrolic-type N (399.0 eV), pyrrolic N (399.9 eV), and graphitic N (401.5 eV), demonstrating the existence of N element in Zn 0.76 Co 0.24 S@N/S-rGO [29].
To investigate the electrochemical performance of Zn 0.76 Co 0.24 S@N/S-rGO as anodes for LIBs, cyclic voltammograms (CV) curves of the Zn 0.76 Co 0.24 S@N/S-rGO electrodes  were first carried out at a scan rate of 0.1 mV s −1 between 0.01-3.0 V. As illustrated in Figure 5a, two peaks centered at 0.5 and 0.2 V in the first cathodic scan correspond to the reduction of Zn 0.76 Co 0.24 S to Zn and Co nanocrystals, accompanied with an irreversible reaction related to the formation of the solid electrolyte interphase (SEI) layer on the surface of the electrode [30]. During anodic scan, the peaks at 1.3 and 0.15 V are attributed to the oxidation of Zn and Co [31]. The second and third CV curves of the Zn 0.76 Co 0.24 S@N/S-rGO electrode almost overlap with each other, which imply the superior stability and excellent reversibility of the redox reactions of the Zn 0.76 Co 0.24 S@N/S-rGO electrode [32].
The typical galvanostatic discharge-charge voltage profiles of the Zn 0.76 Co 0.24 S@N/S-rGO electrode at the current density of 0.1 A g −1 within the voltage range of 0.01-3.0 V (versus Li/Li +) are shown in Figure 5b. The Zn 0.76 Co 0.24 S@N/S-rGO electrode exhibits an initial discharge and charge capacity of 1,346 and 872.2 mA h g −1 , respectively. The irreversible capacity of the Zn 0.76 Co 0.24 S@N/S-rGO electrode observed in the first cycle would be attributed to the formation of SEI layer and some irreversible reaction happened at the phase between metal and Li 2 S which leads to an exceptional Li consumption [33]. In addition, other reasons can also contribute to the irreversible capacity, e.g. some oxygencontaining functional groups on GO nanosheet can consume Li + during cycling [34]. For comparison, Zn 0.76 Co 0.24 S shows a lower CE of about 51% (see Figure S5). This is mainly caused by the larger morphology degradation of the electrode without GO protection during the discharge process [35]. The discharge capacities of the Zn 0.76 Co 0.24 S@N/ S-rGO electrode at the second and third cycles are 1000.9 and 930.5 mA h g −1 , respectively. A high gravimetric capacity of 750.6 mA h g −1 could still be maintained by the Zn 0.76 Co 0.24 S@N/S-rGO electrode over 100 cycles ( Figure S6). In order to reveal superior Li + storage properties of Zn 0.76 Co 0.24 S@N/S-rGO and Zn 0.76 Co 0.24 S, the cycle stability of the composites was further evaluated at a current density of 1.0 A g −1 for 500 cycles (Figure 5c). Obviously, Zn 0.76 Co 0.24 S@N/S-rGO electrode can deliver a high capacity of 1,280 mA h g −1 in the first cycle, while Zn 0.76 Co 0.24 S only delivers 980 mA h g −1 . Even after 50 cycles at 1.0 A g −1 , the Zn 0.76 Co 0.24 S@N/S-rGO electrodes can still deliver high reversible discharge capacities of 804 mA h g −1 . However, the reversible capacities of Zn 0.76 Co 0.24 S decrease quickly from 700 (next cycle) to 300 mA h g −1 (500th cycles). It could be easy to explain that the Zn 0.76 Co 0.24 S nanospheres anode suffer serious volume pulverization during the discharge/charge process, which can result in a rapidly capacity fading. The reason why the capacity raised during the cycling has been evaluated in the previous literature, like transition metal oxides or sulphides [36]. It can be concluded from the result that Zn 0.76 Co 0.24 S anchored on graphene can extremely enhance the cycling performance. On the one hand, the layered structured of rGO can provide the reversible capacity, which is beneficial for the diffusion of Li + ions. On the other hand, the rGO can effectively buffer the volume change during cycling [37].
The rate capabilities of the Zn 0.76 Co 0.24 S@N/S-rGO and Zn 0.76 Co 0.24 S electrodes are presented in Figure 5d. The Zn 0.76 Co 0.24 S@N/S-rGO electrode delivers excellent reversible discharge capacities of 925, 850, 837, 808, 758, and 690 mA h g −1 at current densities of 0.1, 0.2, 0.5, 1, 2, and 5 A g −1 , respectively. When the current density was restored to 0.1 A g −1 , a high reversible capacity of 1,014 mA h g −1 could be achieved, indicating excellent rate capacity of the Zn 0.76 Co 0.24 S@N/S-rGO electrode. In contrast, the discharge capacities of Zn 0.76 Co 0.24 S nanosphere decreased dramatically when we raise the current density from 0.1 to 5 A g −1 . The excellent rate capability of Zn 0.76 Co 0.24 S@N/S-rGO is originated from its unique structure [38]. First, the introduction of hollow Zn 0.76 Co 0.24 S nanospheres could expand the intervals of GO layers, which is beneficial for the interfacial compatibility and fast ion transfer [39]. Second, the size of the Zn 0.76 Co 0.24 S spheres was greatly reduced by their interactions with rGO providing more active site for Li storage [40].
Meanwhile, to further explore charge-transfer kinetics and interfacial resistance, the electrochemical EIS is conducted. Figure S6 shows the typical Nyquist plot of Zn 0.76 Co 0.24 S@N/S-rGO and Zn 0.76 Co 0.24 S electrodes before cycling. The high frequency semi-circle represents the charge transfer resistance (R ct ), while the slope in the low frequency region indicates the Warburg impedance [41]. The R e and R ct values of the Zn 0.76 Co 0.24 S@N/S-rGO electrode are significantly smaller than the corresponding values of the Zn 0.76 Co 0.24 S electrode, suggesting that the Zn 0.76 Co 0.24 S@N/S-rGO electrodes have lower ohmic resistance and faster faradic reaction kinetics than the Zn 0.76 Co 0.24 S electrode [42]. Furthermore, the graphene framework-supported Zn 0.76 Co 0.24 S could further decrease the electrical resistance, which is beneficial for the performance of LIBs [43].
In order to study the electrochemical properties of Zn 0.76 Co 0.24 S@N/S-rGO as anode for SIBs, the materials were evaluated in coin-type half cells at room temperature. The CV curves of the Zn 0.76 Co 0.24 S@N/S-rGO electrode at 0.1 mV s −1 are shown in Figure 6a. During the 1st cathodic process, three cathodic peaks located at 0.2, 0.3, and 0.6 V were observed; these peaks corresponded to the insertion of sodium ions into Zn 0.76 Co 0.24 S@N/S-rGO lattice to form Na x Zn 0.76 Co 0.24 S@N/S-rGO, conversion reactions of Zn 0.76 Co 0.24 S@N/S-rGO into Zn and Co metallic nanoparticles embedded into amorphous Na 2 S matrix, and together with the generation of SEI film because of the irreversible reactions [44]. In the reversible charge process, the anodic peaks at around 0.9 V could be attributing to the transformed form Zn to ZnS and the oxidation of Co [45]. The CV curves of the second and third cycles are almost overlapped with each other, indicating the excellent reversible sodium storage of the Zn 0.76 Co 0.24 S@N/S-rGO [46].
The typical galvanostatic discharge/charge curves of the Zn 0.76 Co 0.24 S@N/S-rGO electrode at 0.1 A g −1 are presented in Figure 6b. The first discharge plateau of Zn 0.76 Co 0.24 S@N/S-rGO electrode is located at around 0.6 V, corresponding to the intercalation of Na + into Zn 0.76 Co 0.24 S@N/S-rGO, which is consistent with the CV results ( Figure 6a). The initial discharge and charge capacities of Zn 0.76 Co 0.24 S@N/S-rGO were 686.1 and 649.5 mA h g −1 , respectively, and the initial coulombic efficiency is 96.4%. The discharge capacity of the subsequent cycles was founded to be 625 mA h g −1 . The main reasons for the capacity loss of the Zn 0.76 Co 0.24 S@N/S-rGO in the first few cycles are mainly caused by the decomposition of electrolyte and development of the formation of SEI film [47]. In addition, sodium can also be consumed by the active oxygen-containing functional group in the rGO. The charge/discharge curves of second and third cycle almost overlap with each other, showing the excellent electrochemical stability and reversibility of Zn 0.76 Co 0.24 S@N/ S-rGO.
Moreover, the long-term cycling of the Zn 0.76 Co 0.24 S@N/ S-rGO and the Zn 0.76 Co 0.24 S at 1 A g −1 are also explored and present in Figure 6c. The discharge capacity is 756 mA h g −1 for Zn 0.76 Co 0.24 S@N/S-rGO and 675 mA h g −1 for pure Zn 0.76 Co 0.24 S, which is lower than that of Zn 0.76 Co 0.24 S@N/ S-rGO. The discharge capacities of Zn 0.76 Co 0.24 S@N/S-rGO and Zn 0.76 Co 0.24 S electrode for the 500th cycles are 605 and 403 mA h g −1 , respectively. The capacity retentions compared with first cycle are 93.1% and 51.7%. The excellent cycling stability of Zn 0.76 Co 0.24 S@N/S-rGO may be attributed to the good conductivity of rGO and the active site of hollow Zn 0.76 Co 0.24 S nanospheres [48]. The poor stability of Zn 0.76 Co 0.24 S can be attributed to the structure collapse without rGO. The rate performances of Zn 0.76 Co 0.24 S@N/S-rGO and Zn 0.76 Co 0.24 S electrodes were also investigated ( Figure 6d). The Zn 0.76 Co 0.24 S@N/S-rGO electrode clearly shows much better rate capability than the Zn 0.76 Co 0.24 S electrode. Reversible specific capacities of 614.4, 601.3, 565.5, 541.3, 516.1, and 478.5 mA h g −1 were obtained at current densities of 0.1, 0.2, 0.5, 1, 2, and 5 A g −1 , respectively. However, the discharge capacities achieved by the Zn 0.76 Co 0.24 S electrodes were 515.3, 479.5, 433.4, 380.6, 316.3, and 237.9 mA h g −1 at current densities from 0.1 to 5 A g −1 . Particularly, despite cycling at high current density of 5 A g −1 , the discharge capacity of Zn 0.76 Co 0.24 S@N/S-rGO can still reach 478.5 mA h g −1 , which is much higher than that of Zn 0.76 Co 0.24 S (237.9 mA h g −1 ). When the current density was restored to 0.1 A g −1 , the capacity of Zn 0.76 Co 0.24 S@N/ S-rGO composites almost recovered to the initial value. The rate capabilities of the Zn 0.76 Co 0.24 S@N/S-rGO are much better than those without GO which can further confirm the superior cycling stability of the Zn 0.76 Co 0.24 S@N/ S-rGO electrodes.
The advantages of the Zn 0.76 Co 0.24 S@N/S-rGO electrode in simultaneously achieving efficient Li + /Na + storage are shown in Figure 7. The superior electrochemical properties can be attributed to the unique structure in exploring the accessible active sites, which can effectively improve the ion/electron transport [49]. The graphene network can protect the Zn 0.76 Co 0.24 S nanospheres, which can increase the active sites between electrolyte and Li/Na ion. Moreover, the excellent electronic conductivity of 3D graphene network can enhance the electron and ion transport during the whole electrode [50]. As a result, Zn 0.76 Co 0.24 S nanospheres integrating graphene network, hollow Zn 0.76 Co 0.24 S nanospheres, and 3D graphene synergistically enhance the electrochemical performance.

Conclusion
In summary, rGO-supported hollow Zn 0.76 Co 0.24 S nanospheres composite has been successfully synthesized through an effective hydrothermal method followed by freeze-drying and calcination. The rGO matrix can increase the conductivity of the hybrid materials, as well as tolerate the fast ion intercalation and extraction of electrodes during cycling process. Meanwhile, the encapsulated hollow Zn 0.76 Co 0.24 S nanospheres can provide more active sites and are beneficial for the transportation of Li/Na ions, which can give rise to the high rate capacity and excellent performance. Benefited from the smart structural and compositional features, Zn 0.76 Co 0.24 S@N/S-rGO-based LIBs and SIBs exhibit high initial discharge capacities, excellent rate capability, and stable cycling performance. As a result, Zn 0.76 Co 0.24 S@N/S-rGO exhibits a high reversible capacity of 605 mA h g −1 for SIBs at 1 A g −1 over 500 cycles. Specifically, even at a high rate of 5 A g −1 , the capacities of LIBs and SIBs can still retain 690 and 478.5 mA h g −1 , respectively. These superior Li/Na storage performances of Zn 0.76 Co 0.24 S@N/S-rGO make it promising anode materials for both LIBs and SIBs.