Combining Zn_0.76Co_0.24S with S-doped graphene as high-performance anode materials for lithium- and sodium-ion batteries

1 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_0.76Co_0.24S nanospheres anchored on rGO as anodes materials of LIBs and SIBs. The obtained Zn_0.76Co_0.24S@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_0.76Co_0.24S@N/S-rGO are mainly attributed to the following advantages. First, the mesoporous hollow-structured Zn_0.76Co_0.24S 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].

2 Experimental section

2.1 Synthesis of Zn_0.76Co_0.24S@N/S-rGO and Zn_0.76Co_0.24S nanospheres

The aqueous suspension of GO was obtained by dispersing graphite flakes into deionized water (via well-known modified Hummers method) and the concentration of GO was 1 mg mL⁻¹ [17]. Zn_0.76Co_0.24S@N/S-rGO composite was synthesized through a facile hydrothermal method. In a typical synthesis of Zn_0.76Co_0.24S@N/S-rGO, the GO solution was sonicated for 1 h and then stirred for half an hour. Then 7.6 × 10⁻⁴ mol of Zinc nitrate and 2.4 × 10⁻⁴ mol of cobalt nitrate hexahydrate were introduced into 35 mL GO solution (solution A). The solution B was prepared by adding 3 × 10⁻⁴ mol l-cysteine and another 35 mL GO solution. After mixing solution B into solution A under continuous stirring for an hour, the obtained mixture was then transferred into 100 mL Teflon-lined autoclave for solvothermal at 180°C for 6 h. The black precipitate produced was centrifuged at 8,000 rpm for 5 min and washed with deionized water and absolute ethanol several times, followed by freeze-dried for further use. Then the dried product was calcinated at 400°C at a heating rate of 2°C min⁻¹ for 3 h under argon atmosphere to obtain the Zn_0.76Co_0.24S@N/S-rGO composite. Pure Zn_0.76Co_0.24S nanospheres were prepared with a similar approach above, except the addition of GO.

In this work, Zn_0.76Co_0.24S nanospheres were uniformly distributed and anchored on N/S-doped reduced graphene oxide (rGO) network through an effective solvothermal method (denoted as Zn_0.76Co_0.24S@N/S-rGO). As illustrated in Scheme 1, GO was dispersed in deionized water and sonicated for half an hour. Then, ZnNO₃·6H₂O, CoNO₃·6H₂O 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_0.76Co_0.24S nanospheres were tightly anchored on the GO network.

Scheme 1

Schematic illustration of the synthesis procedure of Zn_0.76Co_0.24S@N/S-rGO composites.

3 Results and discussions

The morphologies of Zn_0.76Co_0.24S nanospheres and Zn_0.76Co_0.24S@N/S-rGO composite were investigated via scanning electron microscope (SEM) and transmission electron microscope (TEM). The size of Zn_0.76Co_0.24S nanospheres (Figure S1) is obviously larger than that of Zn_0.76Co_0.24S@N/S-rGO, which implies that the introduction of graphene could limit the size of nanospheres. Compared with pure Zn_0.76Co_0.24S nanospheres, the SEM images of Zn_0.76Co_0.24S@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_0.76Co_0.24S nanospheres on rGO can effectively inhibit the aggregation and further growth of Zn_0.76Co_0.24S nanospheres demonstrated by the TEM images (Figure 1 and Figure S1). Figure 1c shows TEM image of Zn_0.76Co_0.24S@N/S-rGO, which displays a perfect hollow-structured Zn_0.76Co_0.24S nanosphere tightly anchored on rGO matrix. The high-resolution TEM (HRTEM) image of Zn_0.76Co_0.24S@N/S-rGO composites in Figure 1d indicates that the Zn_0.76Co_0.24S nanospheres are well-dispersed on the rGO nanosheet and the lattice spacings of 0.312 and 0.269 nm correspond to the (111) and (200) planes of Zn_0.76Co_0.24S@N/S-rGO.

Figure 1

(a) Low and (b) high magnification SEM images of Zn_0.76Co_0.24S@N/S-rGO. (c) TEM images of Zn_0.76Co_0.24S@N/S-rGO. (d) HRTEM image of Zn_0.76Co_0.24S@N/S-rGO.

In order to further investigate the crystallographic structure and graphitization degree of carbon in the as-obtained Zn_0.76Co_0.24S@N/S-rGO composite, XRD and Raman spectra characterization were conducted. As depicted in Figure 2a, all main diffraction peaks of Zn_0.76Co_0.24S@N/S-rGO and Zn_0.76Co_0.24S were in good agreement with the standard patterns of Zn_0.76Co_0.24S (JCPDS no. 47-1656); the peak at 28.6°, 47.6°, 56.4°, and 76.9°, respectively, which are indexed to the (111), (220), (311), and (331) planes of the cubic phase of Zn_0.76Co_0.24S. The sharp peaks indicate the good crystallinity of the as-obtained Zn_0.76Co_0.24S. The small peak at 26.5° in Zn_0.76Co_0.24S@N/S-rGO could be assigned to rGO. No obvious peak related to graphite can be observed in the XRD pattern of Zn_0.76Co_0.24S@N/S-rGO, indicating amorphous carbon in the composite. Moreover, the content of Zn_0.76Co_0.24S in Zn_0.76Co_0.24S@N/S-rGO is calculated to be around 67% in terms of the result of thermogravimetric analysis (Figure S2) [18]. The energy-dispersive X-ray spectroscopy of Zn_0.76Co_0.24S@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_0.76Co_0.24S. 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_0.76Co_0.24S@N/S-rGO and GO samples are given in Figure 2b to better understand the surface layer of Zn_0.76Co_0.24S@N/S-rGO. The broad band located around 1,590 cm⁻¹ corresponds to the E₂g symmetric vibrational mode of graphite-type sp² 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 sp³ carbons [19]. The Raman spectra further confirm the residual carbon in the Zn_0.76Co_0.24S nanospheres, which is generated from the carbonization of organic molecules in the annealing process.

Figure 2

(a) XRD patterns of the Zn_0.76Co_0.24S@N/S-rGO and (b) Raman spectra of GO and Zn_0.76Co_0.24S@N/S-rGO.

The Brunauer–Emmett–Teller (BET) analysis was carried out to evaluate the structural feature and porosity of Zn_0.76Co_0.24S@N/S-rGO and Zn_0.76Co_0.24S. The nitrogen adsorption–desorption isotherms and pore-size distribution curves of Zn_0.76Co_0.24S@N/S-rGO and Zn_0.76Co_0.24S are shown in Figure 3a and b, respectively. The BET-specific surface area was measured to be 40.9 m² g⁻¹ for Zn_0.76Co_0.24S@N/S-rGO and 25.2 m² g⁻¹ for Zn_0.76Co_0.24S. The nitrogen adsorption–desorption isotherm of Zn_0.76Co_0.24S@N/S-rGO was found to coincide with type IV BET classification [20]. As the pore-size distribution curve shows, the pores of Zn_0.76Co_0.24S@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_0.76Co_0.24S.

Figure 3

(a) N₂-sorption-isotherms and corresponding pore-size distribution of the (a) Zn_0.76Co_0.24S@N/S-rGO and (b) Zn_0.76Co_0.24S.

X-ray photoemission spectroscopy (XPS) measurements were carried out to further study the chemical composition and valence states of Zn_0.76Co_0.24S@N/S-rGO. XPS survey spectra of the Zn_0.76Co_0.24S@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 Zn²⁺ (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 Co²⁺, demonstrating the coexistence of Co³⁺ and Co²⁺ 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_0.76Co_0.24S, resulting in the formation of a stable Zn_0.76Co_0.24S@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_0.76Co_0.24S@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_0.76Co_0.24S@N/S-rGO [29].

Figure 4

XPS spectra of the as-synthesized Zn_0.76Co_0.24S@N/S-rGO: (a) survey, (b) Zn 2p, (c) Co 2p, (d) S 2p, (e) C 1s, and (f) N 1s.

To investigate the electrochemical performance of Zn_0.76Co_0.24S@N/S-rGO as anodes for LIBs, cyclic voltammograms (CV) curves of the Zn_0.76Co_0.24S@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_0.76Co_0.24S 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_0.76Co_0.24S@N/S-rGO electrode almost overlap with each other, which imply the superior stability and excellent reversibility of the redox reactions of the Zn_0.76Co_0.24S@N/S-rGO electrode [32].

Figure 5

Lithium ion storage behaviours of Zn_0.76Co_0.24S@N/S-rGO and Zn_0.76Co_0.24S composite electrodes: (a) cyclic voltammetry curves of Zn_0.76Co_0.24S@N/S-rGO between 0.01 and 3 V with a scan rate of 0.1 mV s⁻¹; (b) charge/discharge voltage profiles of Zn_0.76Co_0.24S@N/S-rGO at a rate of 0.1 A g⁻¹; (c) cycling performance of the two different materials at a current density of 1 A g⁻¹; and (d) rate performance of two different electrodes.

The typical galvanostatic discharge–charge voltage profiles of the Zn_0.76Co_0.24S@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_0.76Co_0.24S@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_0.76Co_0.24S@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 Li₂S which leads to an exceptional Li consumption [33]. In addition, other reasons can also contribute to the irreversible capacity, e.g. some oxygen-containing functional groups on GO nanosheet can consume Li⁺ during cycling [34]. For comparison, Zn_0.76Co_0.24S 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_0.76Co_0.24S@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_0.76Co_0.24S@N/S-rGO electrode over 100 cycles (Figure S6).

In order to reveal superior Li⁺ storage properties of Zn_0.76Co_0.24S@N/S-rGO and Zn_0.76Co_0.24S, 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_0.76Co_0.24S@N/S-rGO electrode can deliver a high capacity of 1,280 mA h g⁻¹ in the first cycle, while Zn_0.76Co_0.24S only delivers 980 mA h g⁻¹. Even after 50 cycles at 1.0 A g⁻¹, the Zn_0.76Co_0.24S@N/S-rGO electrodes can still deliver high reversible discharge capacities of 804 mA h g⁻¹. However, the reversible capacities of Zn_0.76Co_0.24S decrease quickly from 700 (next cycle) to 300 mA h g⁻¹ (500th cycles). It could be easy to explain that the Zn_0.76Co_0.24S 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_0.76Co_0.24S 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_0.76Co_0.24S@N/S-rGO and Zn_0.76Co_0.24S electrodes are presented in Figure 5d. The Zn_0.76Co_0.24S@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_0.76Co_0.24S@N/S-rGO electrode. In contrast, the discharge capacities of Zn_0.76Co_0.24S nanosphere decreased dramatically when we raise the current density from 0.1 to 5 A g⁻¹. The excellent rate capability of Zn_0.76Co_0.24S@N/S-rGO is originated from its unique structure [38]. First, the introduction of hollow Zn_0.76Co_0.24S 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_0.76Co_0.24S 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_0.76Co_0.24S@N/S-rGO and Zn_0.76Co_0.24S 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_0.76Co_0.24S@N/S-rGO electrode are significantly smaller than the corresponding values of the Zn_0.76Co_0.24S electrode, suggesting that the Zn_0.76Co_0.24S@N/S-rGO electrodes have lower ohmic resistance and faster faradic reaction kinetics than the Zn_0.76Co_0.24S electrode [42]. Furthermore, the graphene framework-supported Zn_0.76Co_0.24S could further decrease the electrical resistance, which is beneficial for the performance of LIBs [43].

In order to study the electrochemical properties of Zn_0.76Co_0.24S@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_0.76Co_0.24S@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_0.76Co_0.24S@N/S-rGO lattice to form Na _x Zn_0.76Co_0.24S@N/S-rGO, conversion reactions of Zn_0.76Co_0.24S@N/S-rGO into Zn and Co metallic nanoparticles embedded into amorphous Na₂S 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_0.76Co_0.24S@N/S-rGO [46].

Figure 6

Electrochemical performance of Zn_0.76Co_0.24S@N/S-rGO as anodes for SIBs (a) CV curves of Zn_0.76Co_0.24S@N/S-rGO electrode at a scan rate of 0.1 mV s⁻¹ within a potential range of 0.01–3.0 V (vs Na/Na⁺). (b) Discharge/charge voltage curves of the with Zn_0.76Co_0.24S@N/S-rGO electrode at a current density of 1 A g⁻¹. (c) Cycling performance at current density of 1 A g⁻¹. (d) Rate capacity.

The typical galvanostatic discharge/charge curves of the Zn_0.76Co_0.24S@N/S-rGO electrode at 0.1 A g⁻¹ are presented in Figure 6b. The first discharge plateau of Zn_0.76Co_0.24S@N/S-rGO electrode is located at around 0.6 V, corresponding to the intercalation of Na⁺ into Zn_0.76Co_0.24S@N/S-rGO, which is consistent with the CV results (Figure 6a). The initial discharge and charge capacities of Zn_0.76Co_0.24S@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_0.76Co_0.24S@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_0.76Co_0.24S@N/S-rGO.

Moreover, the long-term cycling of the Zn_0.76Co_0.24S@N/S-rGO and the Zn_0.76Co_0.24S at 1 A g⁻¹ are also explored and present in Figure 6c. The discharge capacity is 756 mA h g⁻¹ for Zn_0.76Co_0.24S@N/S-rGO and 675 mA h g⁻¹ for pure Zn_0.76Co_0.24S, which is lower than that of Zn_0.76Co_0.24S@N/S-rGO. The discharge capacities of Zn_0.76Co_0.24S@N/S-rGO and Zn_0.76Co_0.24S 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_0.76Co_0.24S@N/S-rGO may be attributed to the good conductivity of rGO and the active site of hollow Zn_0.76Co_0.24S nanospheres [48]. The poor stability of Zn_0.76Co_0.24S can be attributed to the structure collapse without rGO. The rate performances of Zn_0.76Co_0.24S@N/S-rGO and Zn_0.76Co_0.24S electrodes were also investigated (Figure 6d). The Zn_0.76Co_0.24S@N/S-rGO electrode clearly shows much better rate capability than the Zn_0.76Co_0.24S 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_0.76Co_0.24S 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_0.76Co_0.24S@N/S-rGO can still reach 478.5 mA h g⁻¹, which is much higher than that of Zn_0.76Co_0.24S (237.9 mA h g⁻¹). When the current density was restored to 0.1 A g⁻¹, the capacity of Zn_0.76Co_0.24S@N/S-rGO composites almost recovered to the initial value. The rate capabilities of the Zn_0.76Co_0.24S@N/S-rGO are much better than those without GO which can further confirm the superior cycling stability of the Zn_0.76Co_0.24S@N/S-rGO electrodes.

The advantages of the Zn_0.76Co_0.24S@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_0.76Co_0.24S 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_0.76Co_0.24S nanospheres integrating graphene network, hollow Zn_0.76Co_0.24S nanospheres, and 3D graphene synergistically enhance the electrochemical performance.

Figure 7

Schematic illustration of the enhanced ion/electron transport of the Zn_0.76Co_0.24S@N/S-rGO electrode.

4 Conclusion

In summary, rGO-supported hollow Zn_0.76Co_0.24S 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_0.76Co_0.24S 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_0.76Co_0.24S@N/S-rGO-based LIBs and SIBs exhibit high initial discharge capacities, excellent rate capability, and stable cycling performance. As a result, Zn_0.76Co_0.24S@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_0.76Co_0.24S@N/S-rGO make it promising anode materials for both LIBs and SIBs.