Facile and Controllable Synthesis of CuS@Ni-Co Layered Double Hydroxide Nanocages for Hybrid Supercapacitors.

The synthesis of battery-type electrode materials with hollow nanostructures for high-performance hybrid supercapacitors (HSCs) remains challenging. In this study, hollow CuS@Ni-Co layered double hydroxide (CuS-LDH) composites with distinguished compositions and structures are successfully synthesized by co-precipitation and the subsequent etching/ion-exchange reaction. CuS-LDH-10 with uniformly dispersed CuS prepared with the addition of 10 mg of CuS shows a unique hollow polyhedral structure constituted by loose nanosphere units, and these nanospheres are composed of interlaced fine nanosheets. The composite prepared with 30 mg of CuS addition (CuS-LDH-30) is composed of a hollow cubic morphology with vertically aligned nanosheets on the CuS shell. The CuS-LDH-10 and CuS-LDH-30 electrodes exhibit high specific capacity (765.1 and 659.6 C g-1 at 1 A g-1, respectively) and superior cycling performance. Additionally, the fabricated HSC delivers a prominent energy density of 52.7 Wh kg-1 at 804.5 W kg-1 and superior cycling performance of 87.9% capacity retention after 5000 cycles. Such work offers a practical and effortless route for synthesizing unique metal sulfide/hydroxide composite electrode materials with hollow structures for high-performance HSCs.

Introduction

Today, the energy crisis and environmental sustainability have forced us to develop more advanced energy systems.[1−4] Among the emerging energy storage devices, supercapacitors (SCs) have drawn significant attention because of their excellent cycling stability, quick charge–discharge capability, and ultrahigh power density.[5−7] However, the insufficient energy density dramatically hinders their further commercial application.[8,9] To enhance the energy densities, hybrid supercapacitors (HSCs) have become one of the promising development approaches by combining the excellent energy density of battery-type faradaic electrodes with a carbon negative electrode.[10,11] The intrinsic properties of the battery-type materials are regarded as the vital factor in achieving high-performance HSCs.[12−15]

Transition metal compounds[16−19] are typical positive electrode materials with superior electrochemical performance. Among them, layered double hydroxides (LDHs) are commonly used as electrode materials, benefitting from their numerous redox states and high tunability of multivalent metal cations, as well as unique two-dimensional (2D) lamellar structures.[20−22] Nevertheless, LDHs are often plagued by their deficient conductivity, unacceptable structural stability, and strong stacking propensity.[23−25] Hollow nanostructures with well-defined interior voids are of great interest because of their structural merits of shortened diffusion paths for charge transport and large surface permeability.[26,27] Accordingly, the design of LDH materials with hollow nanostructures is a promising approach to improving their electrochemical performance; however, it is still a huge challenge to create hollow structures with desired electrochemical performance. Zeolitic imidazolate frameworks (ZIFs) are comprised of metal ions and organic ligands.[28,29] ZIFs have tunable structures and can be employed as both metal precursors and sacrificial templates to fabricate well-defined hollow structures.[30−32] Chen et al.[33] successfully prepared hollow NiCo-LDH nanocage materials with excellent electrochemical properties using ZIF-67 as a precursor. Generally, MOF-derived hollow LDH materials have insufficient structural stability and conductivity, resulting in the attenuation of electrochemical properties.[20,34] It is considered that transition metal sulfides (TMSs) with lower energy band gap and higher electrical conductivity can combine with LDHs to effectively meliorate their electrochemical performance.[35−38] Among the various metal sulfides, CuS possessing excellent metal-like conductivity, complex valence states, and controllable structures has attracted considerable attention.[39,40] Moreover, CuS can be prepared with diverse structures, such as nanoplates,[41] nano/microflowers,[42,43] nanocages,[44] and nanospheres,[39] for application in supercapacitors.

Herein, a feasible and effective route for synthesizing novel CuS@Ni-Co layered double hydroxide (CuS-LDH) composite materials with hollow structures is proposed. The key processes involve the synthesis of CuS@ZIF-67s by the growth reaction of ZIF-67 with different amounts of CuS and sequential transformation of CuS@ZIF-67s into CuS-LDHs by reaction with Ni(NO3)2·6H2O. The CuS-LDHs possess various morphologies and structures with different amounts of CuS addition. CuS-LDH-10 with uniformly dispersed CuS shows a unique hollow polyhedral structure constituted by loose nanosphere units, and these nanosphere units are composed of interlaced fine nanosheets, while the vertically aligned NiCo-LDH nanosheets of CuS-LDH-30 are grown on the surface of the CuS shells to create the hollow cubic morphology. Significantly, the CuS-LDH-10 and CuS-LDH-30 electrodes exhibit high specific capacity (765.1 and 659.6 C g–1 at 1 A g–1, respectively) and superior cycling performance. Additionally, the fabricated HSC device (CuS-LDH-10//active carbon) exhibits a maximum energy density of 52.7 Wh kg–1at 804.5 W kg–1 and superior cycling stability (87.9% capacity retention after 5000 cycles).

Results and Discussion

Characterization of Morphology and Structure

The synthesis procedure of CuS-LDH composite materials is illustrated in Scheme . The scanning electron microscopy (SEM) images (Figures S1a and S2a) demonstrate that the prepared Cu2O and CuS have cubic shapes and hollow cubic morphologies, respectively. The X-ray diffraction (XRD) patterns of prepared Cu2O (Figure S1b) and CuS (Figure S2b) can be readily assigned to the reported pure Cu2O (JCPDS 78-2076)[45] and CuS (JCPDS 06-0464), respectively.[46−48] These results demonstrate the successful synthesis of CuS nanoboxes.

Scheme 1

Schematic Illustration of the Preparation of Hollow H-LDH and CuS-LDH Nanocages

The structures and morphologies of the as-prepared ZIF-67, CuS@ZIF-67s, H-LDH, and CuS-LDH materials were detected using SEM, as displayed in Figure . The structures of CuS@ZIF-67s prepared with different amounts of CuS demonstrate significant differences from that of ZIF-67 because of the nucleation effect of CuS, and the CuS nanoboxes are wrapped in CuS@ZIF-67s. ZIF-67 (Figure a) possesses a typical rhombic dodecahedron structure with some cracks and pores on the surface, and its average particle size is approximately 1.5 μm. CuS@ZIF-67-10, CuS@ZIF-67-20, and CuS@ZIF-67-30 (Figure b–d) present distorted rhombic dodecahedron structures, and their average particle sizes are ca. 2.7 μm, 1.2 μm, and 750 nm, respectively. CuS@ZIF-67-10 shows a rough surface with some flake particles, which might be caused by the adsorption of incompletely formed ZIF-67. CuS@ZIF-67-10 has a much larger particle size than that of ZIF-67, while the particle size of CuS@ZIF-67-30 is slightly larger than the size of CuS (450 nm) due to the addition of more CuS nuclei during the growth process of ZIF-67. Hollow H-LDH and CuS-LDH nanocages were synthesized by adding Ni(NO3)2·6H2O to react with ZIF-67 and CuS@ZIF-67s, respectively. According to the SEM images (Figure e), the H-LDH nanocages inherit the polyhedron structure of ZIF-67, and their surfaces are constituted by the vertically arranged nanosheets. CuS-LDH-10 (Figure f) is composed of abundant uniform nanospheres, which are fabricated by the reasonable stacking of NiCo-LDH nanosheets. Moreover, CuS-LDH-20 (Figure g) demonstrates a distorted polyhedron structure and is also composed of nanospheres similar to CuS-LDH-10, but the nanospheres are more compact. Whereas the structure of CuS-LDH-30 is closer to CuS nanoboxes, and the vertically arranged nanosheets grow uniformly on their cubic shell; besides, some damaged structures can also be found in Figure h.

Figure 1

SEM images of (a) ZIF-67, (b) CuS@ZIF-67-10, (c) CuS@ZIF-67-20, (d) CuS@ZIF-67-30, (e) H-LDH, (f) CuS-LDH-10, (g) CuS-LDH-20, and (h) CuS-LDH-30.

The H-LDH and CuS-LDH samples prepared with different amounts of CuS were further analyzed using transmission electron microscopy (TEM) to determine the inherent properties and formation mechanism. The formation mechanism of the ultrathin nanosheet NiCo-LDH is tentatively proposed. Primarily, the protons generated from the hydrolysis of Ni(NO3)2 etch ZIF-67. Subsequently, the released Co2+ is partially oxidized by dissolved oxygen and NO3–. Eventually, the formed Co2+/Co3+ co-precipitates with Ni2+ ions to obtain NiCo-LDH nanosheets.[49] The TEM images (Figure a–d) further confirm the hollow structures with a homogeneous nanosheet array of the as-prepared samples after reacting with Ni(NO3)2·6H2O. H-LDH exhibits a polyhedron structure, which is assembled by the stacking of ultrathin nanosheets, and well retains the morphology and dimensions of the original ZIF-67 crystals (Figure a). With the addition of CuS, CuS-LDH-10 (Figure b) and CuS-LDH-20 (Figure c) could also maintain their respective polyhedron structures, indicating the stronger template effect of CuS@ZIF-67-10 and CuS@ZIF-67-20 than CuS during the transformation process. The TEM image exhibits that CuS-LDH-10 possesses loose spherical subunits composed of interlaced fine nanosheets. By contrast, CuS-LDH-30 shows similar morphology to the CuS nanoboxes, and the vertically aligned NiCo-LDH nanosheets are grown on the surface of the CuS shells, which demonstrates that the growth of nanosheets is dependent upon the CuS template and several CuS nanocages might be contained in CuS@ZIF-67s during the growth process (Figure d). Energy-dispersive X-ray spectroscopy (EDS) mappings of CuS-LDHs can prove the presence of Ni, Co, Cu, S, and O elements (Figures e,f andS3); besides, Cu and S are homogeneously dispersed in the samples, implying that the CuS nanoboxes might be broken into fragments. Thus, during the growth of ZIF-67, single or multiple CuS nanoboxes might be included inside the ZIF crystals. With a relatively low addition amount (i.e., 10 mg and 20 mg) of CuS, the sizes of CuS@ZIF-67-10 and CuS@ZIF-67-20 are much larger than the CuS nanoboxes; thus, the formed CuS-LDH-10 and CuS-LDH-20 could retain the similar polyhedron structures of the respective precursors. Furthermore, the particle size of CuS@ZIF-67-30 is slightly larger than that of CuS nanoboxes; thus, the nanosheets are more likely generated relying on the morphology and structure of CuS. As a result, CuS-LDH-30 could possess cubic, multicubic stacked, and broken cubic structures due to the fragmentation of the CuS nanoboxes. The existence of irregular cubic structures further identifies the uniform distribution of CuS flakes in CuS-LDH-10 and CuS-LDH-20, which could enhance structural stability. These structural and compositional characteristics of CuS-LDHs could provide a more sufficient contact area, facilitate the ion transfer between electrolyte and active material, and enhance the electroconductivity and structural stability.

Figure 2

TEM images of (a) H-LDH, (b) CuS-LDH-10, (c) CuS-LDH-20, and (d) CuS-LDH-30 and EDS mappings of (e) CuS-LDH-10 and (f) CuS-LDH-30.

The as-prepared samples were characterized using XRD to analyze their phase compositions, as displayed in Figure a. Definite characteristic diffraction peaks can be observed at 2θ = 11.06, 21.86, 34.12, 59.88, and 71.08 indexed to the (003), (006), (102), (110), and (202) plane reflections of the typical NiCo-LDH materials (JCPDS 46-0605).[50,51] CuS-LDHs show the diffraction peaks of the hydrotalcite-like NiCo-LDH phase; besides, several new distinct diffraction peaks at 27.68, 29.34, 31.8, 48.04, and 52.6 indexed to (101), (102), (103), (110), and (108) crystal planes of CuS (JCPDS 06-0464) appear,[46−48] indicating that the CuS-LDH composite materials contain both the NiCo-LDH and CuS phases.

Figure 3

Analysis of CuS-LDH-10. (a) XRD patterns; (b) X-ray photoelectron spectroscopy (XPS) spectrum; and (c) Ni 2p, (d) Co 2p, (e) Cu 2p, and (f) S 2p and O 1s high-resolution XPS spectra.

The elemental compositions and chemical states of as-synthesized samples were studied via X-ray photoelectron spectroscopy (XPS). The XPS spectrum of CuS-LDH-10 (Figure b) also confirms the presence of Ni, Co, Cu, S, and O. As shown in Figure c, the Ni 2p spectra of CuS-LDH-10 can be fitted well with two shake-up satellites (denoted as “Sat.”) and a significant spin–orbit bimodal located at 855.6 eV (Ni 2p3/2) and 873.1 eV (Ni 2p1/2), which are designated to the signals of Ni2+.[52,53] The Co 2p spectrum (Figure d) is deconvolved into two spin–orbit doublet peaks and two shake-up satellites. The deconvoluted peaks at 781.1 and 796.4 eV correspond to Co3+, and the peaks at 782.1 and 798.1 eV are attributed to the Co2+ valance state.[54−56] The deconvoluted Cu 2p spectrum (Figure e) presents two characteristic peaks, and the distinguished peaks at 932.1 eV (Cu 2p3/2) and 953.8 eV (Cu 2p1/2) are related to Cu2+ in CuS.[57,58] In the S 2p spectra (Figure f), the two diffraction peaks at 161.4 and 162.8 eV are associated with S 2p3/2 and S 2p1/2 of CuS,[57,58] respectively, and the two diffraction peaks of the O 1s spectrum are attributed to the signals of O2–. In addition, Figures S4 and S5 show that CuS-LDH-20 and CuS-LDH-30 have similar elemental compositions and chemical states to those of CuS-LDH-10.

The specific surface area and porous properties of the samples were obtained via a N2 adsorption/desorption test. All the samples show type-IV isotherms with H3 hysteresis loops (Figure a), implying their mesoporous structure.[59,60]Table S1 shows the detailed porosity parameters of the samples. The Brunauer–Emmett–Teller (BET) specific surface area of H-LDH, CuS-LDH-10, CuS-LDH-20, and CuS-LDH-30 is 42.6, 50.7, 51.6, and 56.6 m2 g–1, respectively. The pore size distribution (PSD, Figure b) further reveals the mesoporous features of H-LDH, CuS-LDH-10, CuS-LDH-20, and CuS-LDH-30, and their main pore sizes are ca. 3.84, 3.86, 3.80, and 3.82 nm, respectively. The mesoporous structure could accelerate the transformation of electrolyte ions and alleviate the volume change during the energy storage process. Moreover, the larger specific surface area of CuS-LDHs could enhance the electroactive sites and accessible surface of the electrolyte ions, thus resulting in the improved electrochemical performance of the electrode materials.[61,62]

Figure 4

N2 sorption isotherms (a) and PSD curves (b) of H-LDH, CuS-LDH-10, CuS-LDH-20, and CuS-LDH-30.

Electrochemical Properties of the As-Prepared Materials

The supercapacitor performance of the synthesized samples was studied using a three-electrode system in a 2 M KOH electrolyte. Figure a displays the cyclic voltammetry (CV) curves of H-LDH, CuS, CuS-LDH-10, CuS-LDH-20, and CuS-LDH-30 at the scan rate of 10 mV s–1 in the potential window of 0–0.6 V (vs Hg/HgO). Obviously, all of the CV curves present a couple of redox peaks with good symmetry, demonstrating the existence of redox reactions on the electrodes and their excellent redox reversibility.[63] CuS-LDH-10 possesses the largest peak current and CV loop area compared with H-LDH, CuS, CuS-LDH-20, and CuS-LDH-30, indicating that CuS-LDH-10 possesses the highest specific capacity.[64−66] The redox reactions related to CuS and NiCo-LDH could be described as follows[39,50]Figure b reveals the galvanostatic charge–discharge (GCD) curves of H-LDH, CuS, CuS-LDH-10, CuS-LDH-20, and CuS-LDH-30 at 1 A g–1. The GCD curves of all samples are nonlinear with obvious potential plateaus; the approximately vertical part is parallel to the capacitive behavior, and a part of the potential plateaus belongs to the faradaic behavior, implying the typical battery-type features of as-prepared materials.[27] The calculated specific capacity values of H-LDH, CuS, CuS-LDH-10, CuS-LDH-20, and CuS-LDH-30 at 1 A g–1 are 612.5, 53.9, 765.1, 695.0, and 659.6 C g–1, respectively. CuS-LDH-10 shows higher capacity than most previously reported battery-type materials, as displayed in Table S2. Additionally, Figure c reveals the GCD curves of CuS-LDH-10 from 1 to 10 A g–1. The specific capacity of CuS-LDH-10 at 1, 2, 3, 5, and 10 A g–1 is 765.1, 745.9, 731.4, 703.9, and 650.7 C g–1, respectively. CuS-LDH-10 can achieve the capacity retention of 85.05% from 1 to 10 A g–1. The GCD curves and specific capacity of H-LDH, CuS, CuS-LDH-20, and CuS-LDH-30 are presented in Figures S6 and5e, respectively. The capacity retention of H-LDH, CuS, CuS-LDH-20, and CuS-LDH-30 is 51.48, 68.09, 63.53, and 66.49% from 1 to 10 A g–1. Figure d shows the CV curves of CuS-LDH-10 from 1 to 50 mV s–1, and a pair of clearly defined redox peaks are more obvious at low scan rates. Furthermore, due to the polarization effect of the electrode, the cathodic peaks transfer to lower potential, and the anodic peaks move toward higher potential with increasing scan rate.[67] Similar CV curves of H-LDH, CuS-LDH-20, and CuS-LDH-30 are found in Figure S7.

Figure 5

(a) CV curves at 10 mV s–1, (b) GCD curves at 1 A g–1, (c) GCD curves of CuS-LDH-10, (d) CV curves of CuS-LDH-10, (e) specific capacities, (f) Nyquist plots, (g) cycling performance of as-prepared samples, and (h) the schematic of charge transfer of CuS-LDH-10.

The internal resistances and reaction kinetics of the electrode materials were evaluated using electrochemical impedance spectra (EIS). The corresponding Nyquist plots and equivalent circuits are displayed in Figure f. The value of equivalent series resistance (Rs) is equal to the intercept of the fitting curve with the real axis in the high-frequency section. The measured semicircle diameter represents charge transfer resistance (Rct), and the slope of the line in the low-frequency section could indicate the ion diffusion resistance (Rw).[68] The Rs values of CuS-LDH-10, CuS-LDH-20, and CuS-LDH-30 are 0.84, 0.87, and 0.88 Ω, respectively, which are smaller than that of H-LDH (0.95 Ω). Besides, CuS-LDH-10 possesses the semicircle with the smallest diameter and a straight line with the largest slope, suggesting the lowest Rct and Rw. Such results indicate that CuS-LDH-10 has the smallest electrolyte ion transfer resistance and electrochemical reaction resistance.

The cycling performance of the prepared electrodes was evaluated using GCD at 10 A g–1 (Figure g). After 5000 cycles, CuS-LDH-10 and CuS-LDH-30 have capacity retention values of ca. 82.9 and 87.3%, respectively, which are significantly higher than that of H-LDH (61.1%). It can thus be proved that CuS-LDH-10 and CuS-LDH-30 possess excellent cycling stability owing to the advantages of the composition and structure of the CuS-LDH composite materials. CuS-LDH-10 can retain the nanosheet structure, but some damaged and collapsed structures can also be found, as shown in Figure S8.

The superior electrochemical properties of the CuS-LDH-10 composite should be associated with its unique structures and compositions, and the charge transfer of CuS-LDH-10 is demonstrated in Figure h. First, the CuS-LDH composite material with uniform distribution of CuS can enhance structural stability and electrochemical conductivity. Second, the hollow CuS-LDH-10 material possesses loose spherical subunits composed of interlaced fine nanosheets, which could provide a more sufficient contact area and facilitate the ion transfer between electrolyte and active material. Finally, the high specific surface area and the mesoporous feature can promise the rapid transport of ions and charges, as well as buffer the volume change during the energy storage process. Thus, the CuS-LDH-10 composite electrode material can realize excellent electrochemical performance.

The electrochemical reaction kinetics of electrode material could be analyzed by a CV test. The relationship between peak currents (i) and scan rate (ν) could be calculated by the following equation[69,70]where a and b are both undetermined constants. The b value is evaluated by the slope of the fitted line of log(ν) vs log(i), as displayed in Figure a. Herein, b = 0.5 represents the electrochemical reactions of the electrode dominated by the diffusion-controlled process, while b = 1 indicates totally surface capacitive behaviors of the electrode material.[69,70] The corresponding b value for anodic peaks of CuS-LDH-10 is 0.62, demonstrating that the material is affected by both surface capacitive behaviors and the diffusion process during the energy storage process. Additionally, the respective contribution for the full capacity is further calculated by eq .[19]where k1ν and k2ν1/2 represent the capacitive and diffusion contributions, respectively. As shown in Figure b, the ratios of the capacitive contribution of the CuS-LDH-10 electrode are 54.5, 57.1, 64.1, and 76.4% at scan rates of 1, 2, 5, and 10 mV s–1, respectively.

Figure 6

Linear relation of log(i) with log(ν) (a) and ratios of capacitive and diffusion contributions (b) of CuS-LDH-10.

Electrochemical Properties of the CuS-LDH-10//Active Carbon HSC Device

To explore the practical performance of the prepared CuS-LDH-10 material, a HSC was assembled using CuS-LDH-10 as the cathode and active carbon (AC) as the anode in a 2 M KOH electrolyte. Figure S9a,b shows the electrochemical measurements of AC within −1.0 to 0 V. The specific capacitances of AC are calculated as 187.2, 168.2, 156.4, 150.7, 149.3, and 147.6 F g–1 at 1, 2, 5, 10, 15, and 20 A g–1, respectively. Likewise, Figure S9c presents the CV curves of AC negative material and CuS-LDH-10 positive material; thus, the voltage of the fabricated HSC could be increased to 1.6 V. To validate such a claim, the CV curves of the HSC (Figure a) were tested from 0–1.0 to 0–1.8 V. It could be seen that the assembled HCS can maintain stable operational voltage up to 1.6 V without an obvious oxygen evolution reaction.

Figure 7

Electrochemical performance of the CuS-LDH-10//AC HSC device: (a) CV curves at different voltage windows at 10 mV s–1, (b) CV curves, (c) GCD curves, (d) specific capacity at different current densities, (e) cycling performance, and (f) Ragone plots.

Figure b demonstrates the CV curves of the tested HSC at various scan rates; the CV curves combine the characteristics of both capacitive and faradaic behaviors. They can maintain similar shapes without obvious deformation from 5 to 100 mV s–1, demonstrating the high reversibility and superb rate capability of the HSC device. All of the GCD curves (Figure c) have good symmetry, indicating the excellent coulombic efficiency and redox reversibility of the device. The calculated specific capacities of the HSC device (Figure d) are 237.3, 208.1, 165.7, 140.4, 121.5, and 107.3 C g–1 at 1, 2, 4, 6, 8, and 10 A g–1, respectively. The cycle performance of the CuS-LDH-10//AC supercapacitor device was evaluated using GCD at 10 A g–1, as displayed in Figure e. The result exhibits that the CuS-LDH-10//AC device delivers 87.9% capacity retention after 5000 cycles and maintains almost 100% coulombic efficiency, proving the excellent cycle performance and the charge–discharge efficiency of the as-prepared HSC.

The Ragone plot (Figure f) of the assembled HSC demonstrates a maximum energy density of 52.7 Wh kg–1 at a power density of 804.5 W kg–1, and the energy density can be maintained at 23.8 Wh kg–1 even at a power density of 8482.4 W kg–1. The properties of the assembled HSC are better than those of previous reports, such as CoS/Ni-Co LDH//AC,[71] Ni(OH)2//AC,[72] Ni-Co LDH//AC,[73] NiCo-LDH@NCF//AC,[74] C/NiCo2S4//AC,[75] and NiCo2S4@graphene//porous carbon.[76] Consequently, the assembled HSC has excellent electrochemical performance and enormous potential in practical applications of energy storage devices.

Conclusions

Hollow CuS-LDH composite materials were synthesized by an effective and facile strategy. Different morphologies and structures were successfully fabricated according to the additive amounts of CuS. CuS-LDH-10 with uniformly dispersed CuS prepared using CuS@ZIF-67-10 as the precursor and a sacrificial template shows a unique hollow polyhedral structure constituted by loose nanosphere units, and these nanosphere units are composed of interlaced fine nanosheets, while the vertically aligned NiCo-LDH nanosheets of CuS-LDH-30 are grown on the surface of the CuS shells to create the hollow cubic morphology. The CuS-LDH-10 and CuS-LDH-30 electrodes possess excellent electrochemical properties, exhibiting high capacity performance (765.1 and 659.6 C g–1 at 1 A g–1, respectively), superior rate capability, and high cycle stability. The assembled hybrid supercapacitor (CuS-LDH-10//AC) delivers a maximum energy density of 52.7 Wh kg–1 at 804.5 W kg–1 and exhibits good cycling performance (87.9% capacity retention after 5000 cycles). The present work may offer facile and effective routes for the exploitation of unique hollow metal sulfide/hydroxide composite electrode materials for high-performance energy storage devices.

Experimental Section

Preparation of CuS Nanoboxes

The Cu2O nanocubes were crafted using a procedure similar to that reported in the literature with minor modifications.[77] Typically, 30.0 mL of NaOH aqueous solution (2.0 mol L–1) was dropped into 300 mL of CuCl2·2H2O aqueous solution (0.01 mol L–1) with stirring for 0.5 h. Then, 30 mL of ascorbic acid solution (0.6 mol L–1) was blended into the mixture and stirred for another 3 h. After washing and drying, the obtained precipitate was Cu2O nanoboxes. The above procedures were carried out in a water bath at 50 °C. Next, 80 mg of as-prepared Cu2O nanocubes was dispersed in 60 mL of methanol, and 4 mL of Na2S solution (0.1 mol L–1) was then mixed. The precipitate was centrifuged and washed with C2H5OH after stirring for 10 min. Likewise, the as-obtained Cu2O@CuS nanocubes were dispersed in 60 mL of CH3OH and mixed with 10 mL of HCl (2 mol L–1). The mixture was stirred for 30 min to collect the CuS nanoboxes after a centrifugal rinse with C2H5OH and drying at 60 °C overnight.

Preparation of CuS@ZIF-67s

Typically, 10 mg of CuS nanoboxes was dispersed in 20 mL of methanol/H2O solvent (volume ratio = 1: 1). Then, 1 mmol of Co(NO3)2·6H2O was poured into the solution with stirring for 30 min to form solution A. Approximately 8 mmol of C4H6N2 was dissolved in 20 mL of H2O with stirring for 30 min to form solution B, which was then poured into solution A with stirring for another 15 min. After 5 h of aging, the precipitate was centrifuged and washed with C2H5OH and dried at 60 °C overnight to obtain CuS@ZIF-67-10. The as-prepared samples for 20 mg and 30 mg of CuS were labeled as CuS@ZIF-67-20 and CuS@ZIF-67-30, respectively. In addition, ZIF-67 was prepared using the same process without CuS.

Preparation of CuS-LDHs

Typically, 80 mg of CuS@ZIF-67-10 was uniformly dispersed in 20 mL of C2H5OH, followed by the addition of 10 mL of C2H5OH containing 320 mg of Ni(NO3)2·6H2O. After stirring for 1 h, CuS-LDH-10 was collected by centrifugal washing with C2H5OH and drying at 60 °C overnight. H-LDH (ZIF-67), CuS-LDH-20 (CuS@ZIF-67-20), and CuS-LDH-30 (CuS@ZIF-67-30) were also prepared successfully using a similar procedure.