Facile synthesis of mesoporous Ni_xCo_9−xS₈ hollow spheres for high-performance supercapacitors and aqueous Ni/Co–Zn batteries

Abstract

Porous micro/nanostructure electrode materials have always contributed to outstanding electrochemical energy storage performances. Co₉S₈ is an ideal model electrode material with high theoretical specific capacity due to its intrinsic two crystallographic sites of cobalt ions. In order to improve the conductivity and specific capacitance of Co₉S₈, nickel ions were introduced to tune the electronic structure of Co₉S₈. The morphology design of the mesoporous hollow sphere structure guarantees cycle stability and ion diffusion. In this work, Ni_xCo_9−xS₈ mesoporous hollow spheres were synthesized via a facile partial ion-exchange of Co₉S₈ mesoporous hollow spheres without using a template, boosting the capacitance to 1300 F g⁻¹ at the current density of 1 A g⁻¹. Compared with the pure Co₉S₈ and Ni-Co₉S₈-30%, Ni-Co₉S₈-60% exhibited the best supercapacitor performance, which was ascribed to the maximum Ni ion doping with morphology and structure retention, enhanced conductivity and stabilization of Co³⁺ in the structure. Therefore, Ni/Co–Zn batteries were fabricated by using a Zn plate as the anode and Ni-Co₉S₈-60% as the cathode, which deliver a high energy density of 256.5 W h kg⁻¹ at the power density of 1.69 kW kg⁻¹. Furthermore, the Ni/Co–Zn batteries exhibit a stable cycling after 3000 repeated cycles with capacitance retention of 69% at 4 A g⁻¹. This encouranging result might provide a new perspective to optimize Co₉S₈-based electrodes with superior supercapacitor and Ni/Co–Zn battery performances.

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1 Introduction

In recent years, as an important transition metal sulfide, Co₉S₈ has been regarded as one of the most attractive electrode materials with a high theoretical specific capacity (5449 F g⁻¹ at 0.45 V),¹ low cost and non-toxic nature, however, the presence of sluggish ion transport kinetics and low conductivity limits its application in the energy storage field. Thus a lot of efforts have been devoted to improving the conductivity by conductive metals or carbonaceous supports,^2–6 ion-doping,⁷ constructing hierarchical composites,⁸ graphene hybridization,^9,10 carbon coating,¹¹ and constructing hetero-phase interfaces.^12–14 In addition to consideration of the conductivity, the electrochemical performance is also dependent on the accessible surface area of transition metal based micro/nanomaterials,^15–17 thus a lot of effect has been devoted to synthesizing electrode materials with large surface areas and porous structures. For example, the reported morphology and synthesis strategies include ion exchange reaction,⁶ microwave irradiation,¹⁸ sacrificial metal salt hard templates¹⁹ and derivation from metal–organic framework precursors via a sulfuration process.^20,21 Considering the complexity and multiple steps of the template transformation method, it is worth developing a simple method without a template to obtain hollow sphere structures.

In general, the mesoporous hollow structure can alleviate the volumetric expansion, accommodate large internal and external surface area and facile electrolyte ions diffusion reaction.^22–24 For example, Zheng reported the metallic hollow Co₉S₈ sphere can serve as sulfur hosts in Li–S batteries and promote interfacial redox reaction kinetics, which present excellent rate performance and high stability.²⁵ Feng designed hierarchical Co₉S₈@carbon hollow microspheres via a solvothermal reaction and subsequent annealing process, which also exhibited remarkable sodium storage performance with remarkable cycling stability.²⁶ Li reported the synthesis of hollow spherical Co₉S₈ using presynthetic silica hollow spheres template to construct Co–SiO₂ precursor and subsequent hydrothermal sulfurizing process.²⁷ Guo et al. using bacteria as template via hydrothermal and further calcination process to fabricate porous Co₉S₈/CoS/C submicronspheres.²⁸ Thus a facile one-step method to prepare Co₉S₈ porous spheres especially mesoporous hollow architecture is appreciated.

Transition metal sulfides especially porous hollow structures are considered as promising electrode materials for electrochemical energy storage devices (EESDs) owing to their relatively abundant, low cost, non-toxic nature, multiple electrochemical reaction sites with high capacity.^29,30 Alkaline aqueous EESDs, such as supercapacitor and Ni/Co–Zn batteries, have many merits consist of environmental friendliness and quickly charge–discharge speed attribute to using high ionic conductance of OH⁻ anions as the charge carries, thus alkaline aqueous EESDs have great potential applications in energy storage field.^31,32 Especially, Co-based sulfides were considered to be good cathodes with high theoretical capacity and operating voltage, which attracted extensive research interests.³³ Extensive exploring micro/nanostructure is help to increase the capacity output and rate capability of cathode.³⁴ The structure of Co₉S₈ is a typical structure with the octahedral site of Co³⁺ and tetrahedral site of Co²⁺. Therefore, doping Ni ions into Co₉S₈ is a promising strategy to increase the Co³⁺ and Ni³⁺ content in the octahedral sites of Co₉S₈ lattices, which effectively synergistic increase the capacity and cycling stability of supercapacitor and Co–Ni//Zn battery. Nevertheless, the investigation of Ni_xCo_9−xS₈ as cathode material in alkaline Ni–Zn battery is rarely reported till now.

Herein, a facile cation exchange approach to synthesize mesoporous hollow structured Ni_xCo_9−xS₈ using the solvothermal synthesized Co₉S₈ microspheres as precursors. The nickel ion doping improves the conductivity of parent Co₉S₈ and promotes electrochemical reaction kinetics. The Ni-Co₉S₈-0.6 shows a battery-type faradaic electrode, which can offer additional charge storage capacity via surface redox processes, e.g., 1300 F g⁻¹ at 1 A g⁻¹, compared with the pure Co₉S₈ electrode (430 F g⁻¹ at 1 A g⁻¹). Furthermore, we exhibit a novel Ni/Co–Zn battery utilizing Ni-Co₉S₈-0.6 mesoporous hollow microspheres as cathode material, due to the high content Co³⁺ ions of Ni-Co₉S₈-0.6, the voltage window of Ni/Co–Zn battery can achieve 1.93 V vs. Zn. These special designs are helped to improve the kinetic property and rate property of the cathode material. This new Ni/Co–Zn battery shows a high power density of 1.69 kW kg⁻¹ at the energy density of 256.46 W h kg⁻¹.

2 Experimental

2.1 Preparation of Ni_xCo_9−xS₈ mesoporous hollow microspheres

To prepare Co₉S₈ microspheres, 0.1 mmol of cobalt acetate tetrahydrate was dissolved in a mixture solution of N,N-dimethylformamide (4 mL), tetramethyl-ethylenediamine (3 mL), and ethanolamine (1 mL) under strong stirring for 0.5 h. Afterwards, 0.1 mmol of thioacetamide was added and further stirred for another 0.5 h. Then the solution was transferred to a 20 mL autoclave and heated at 180 °C for 12 h. The black precipitate was collected by centrifugation and dried at 60 °C for 30 min.

For the synthesis of Ni-Co₉S₈-30% and Ni-Co₉S₈-60% microspheres, the equal amounts (0.03/0.06 mmol) of nickel acetate and thiourea was added to the pre-dispersed aqueous solution (6 mL) containing the as-prepared Co₉S₈ microspheres, then 2 mL of ethylene glycol were added with vigorous stirring, subsequently heated to 160 °C and kept for 4 h. The final samples were washed and dried in vacuum at 60 °C for 30 min. Herein, we use Ni-Co₉S₈-0.3 and Ni-Co₉S₈-0.6 to denote the 30% and 60% Ni ions added in the reaction system.

2.2 Material characterization

The resultant phase of the mesoporous Co₉S₈, Ni-Co₉S₈-0.3 and Ni-Co₉S₈-0.6 hollow microspheres was determined by X-ray diffraction (XRD) on a PANalytical X’ Pert operated at 40 kV and 40 mA. The morphology and microstructure were studied by SEM and TEM technique. The composition and chemical states of the samples was further analyzed by energy-dispersive X-ray spectroscopy (EDS) as well as X-ray photoelectron spectroscopy (XPS, ESCALab, 250XI), respectively. The specific surface area of the samples was measured by N₂ adsorption analyzer (Gemini VII 2390).

2.3 Cathode electrode preparation and measurement

For preparation supercapacitor and alkaline Ni–Zn battery cathode, the electrode slurry was prepared by mixing 16 mg of the Ni_xCo_9−xS₈ powders, 2 mg of acetylene black, and 2 mg of polyvinylidene-fluoride with 110 μL of N-methylpyrrolidone. Then, the slurry was carefully coated onto the pretreated Ni foam with the area of 1 cm × 1 cm, and dried in vacuum at 80 °C for 10 h. The specific capacitance C_s of supercapacitor was calculated from following equation:
where I, Δt, m, ΔV is the current density, discharge time, the mass of active materials, and the potential window, respectively.

The alkaline Ni–Zn battery tests were recorded on an electrochemical workstation (CHI 660E) using two electrode system in which Zn plate as reference and counter electrode, and Ni-Co₉S₈-0.6 as working electrode, 2 M KOH and 20 mM Zn(Ac)₂ were used as mixture electrolytes. AC impendence tests with a platinum coil counter electrode and a saturated calomel electrode as reference electrode. The electrochemical impendence plots were collected at open circuit potential at the frequency range of 0.01 Hz–1 MHz.

3 Results and discussion

Scheme 1 shows the one-step synthesis of unique mesoporous Co₉S₈ hollow microspheres via a mixed solvothermal route, then Ni-doped Co₉S₈ can be obtained by a cation-exchange method with introducing 0.03 or 0.06 mmol nickel acetate and thiourea in a water–ethylene glycol system. Herein, we use Ni-Co₉S₈-0.3 and Ni-Co₉S₈-0.6 to denote the 30% and 60% Ni ions added in the reaction system, and aim to investigate the role of Ni doping in promoting the supercapacitor properties of Co₉S₈ particles.

Scheme 1 The synthesis procedure of Ni_xCo_9−xS₈ series hollow spheres.

The morphology and composition of Co₉S₈ mesoporous hollow spheres were characterized by SEM and EDS. Fig. S1a† and 1a show SEM images of the as-prepared Co₉S₈ microspheres at low and high magnification, it is clearly seen that microspheres assembled by numerous interconnected nanosheets with large inner cavity, after introducing Ni ions, the hollow sphere morphology can be well preserved with thinner size and more closely stacked nanosheets. The as-prepared Co₉S₈ microspheres exhibited an average diameter of ∼2.65 μm (Fig. S1b†). The cracked microspheres provide insights into the hollow interior and the assembly structure. After the Ni ions doping, the microsphere size decreases to from 2.34 to 2.29 μm (Fig. 1c, e, and S1†), which may due to the breakage of the big microspheres via the second step of Ni ions exchange. As shown in the EDS element mapping (Fig. 1d), the cobalt and sulfur elements are homogenous distributed in the Co₉S₈ microspheres. The intensity of Ni element mapping raised following the increase of Ni content in Co₉S₈ matrics (Fig. 1d and f).

Fig. 1 SEM images and EDS element mapping of (a and b) Co₉S₈, (c and d) Ni-Co₉S₈-0.3, (e and f) Ni-Co₉S₈-0.6 hollow microspheres. (The scale bar of b, d, f is 800, 500 and 600 nm).

The TEM image of a typical Ni-Co₉S₈-0.6 hollow microsphere was shown in Fig. 2a, which shows a sharp contrast between the inner cavity and cross stacking nanosheets shell. The special microstructure not only avoid the aggregation and structural collapse, but also provide more surface and inner exposed active sites, which help to achieve desirable rate and cycling performance due to the fast ion diffusion. The lattice interplanar spacing measured to be ∼0.295 nm, which matched well with the (311) plane of cobalt pentlandite Co₉S₈ (Fig. 2b), indicating the structure of Co₉S₈ was stable in moderate Ni ions doping.

Fig. 2 TEM images of Ni-Co₉S₈-0.6 hollow spheres. (a) A single Ni-Co₉S₈-0.6 hollow sphere, (b) HRTEM image.

The phase purities of as-synthesized microspheres were further detected by X-ray power diffraction (Fig. 3a), the diffraction peaks of the three samples match well with the cubic Co₉S₈ phase. The XRD patterns of the three samples are similar, for example, the obtained XRD of Co₉S₈ mesoporous microspheres shown three obvious diffraction peaks at ∼29.8, 47.5 and 52.1°, which corresponded to the (311), (511), and (400) plane of Co₉S₈ (JCPDS no: 03-65-6801). Because the Co ion radius is larger than the Ni ion, the diffraction peaks of Ni ions doped microspheres especially Ni-Co₉S₈-0.6 present a slight moving to higher angle of 2 Theta than pure Co₉S₈, and the calculated d-spacing of (311) plane from Scherrer formula is 0.296 nm, which is lower than that of pure Co₉S₈ (0.299 nm). The XRD result is almost consistent with the HRTEM in Fig. 2b. The energy-dispersive X-ray spectroscopy (EDS) results indicated the presence of Co, S and Ni elements in the series samples. The ratio of Co/S in Co₉S₈ is 9 : 8, which is consistent with the stoichiometry of Co₉S₈. The Ni/Co/S ratio of Ni-Co₉S₈-0.3 and Ni-Co₉S₈-0.6 samples is 1.4 : 7.6 : 6 and 2.8 : 6.2 : 6.5, respectively (Fig. S2†). Therefore, both XRD and EDS results confirmed the pure phase of the Ni doped series of Ni_xCo_9−xS₈. To analyse the surface area and porous structure of the series of hollow spheres, N₂ adsorption–desorption isotherms were conducted. The hysteresis loop for Co₉S₈ located above P/P₀ = 0.8, indicating the presence of mesoporous structure (Fig. 3b). The inset is the Barrett–Joyner–Halenda (BJH) pore distribution for pure Co₉S₈, the mean pore size is ∼47 nm with the BET surface area of 65.92 m² g⁻¹. The surface area of Co₉S₈ (65.92 m² g⁻¹) is larger than Ni-Co₉S₈-0.3 (54.18 m² g⁻¹) and Ni-Co₉S₈-0.6 (55.73 m² g⁻¹) samples as shown in Fig. 3c and d. In addition, the mean pore distribution for Ni-Co₉S₈-0.3 and Ni-Co₉S₈-0.6 decreased to 29.21 and 18.08 nm after nickel ions exchange into Co₉S₈ structure. Thus, the porous structure facilitates electrolyte diffusion and electron transport during the redox electrochemical reaction.

Fig. 3 (a) XRD patterns of Ni_xCo_9−xS₈ series hollow spheres. Nitrogen adsorption–desorption isotherms and the corresponding pore size distributions (inset) of (b) Co₉S₈, (c) Ni-Co₉S₈-0.3, and (d) Ni-Co₉S₈-0.6 hollow spheres.

Furthermore, XPS in Fig. 4 also indicated the existence of Co, S, and Ni elements in the as-obtained Ni-Co₉S₈-0.3 and Ni-Co₉S₈-0.6 microspheres and the variation of surface chemical state of Co after Ni ion doping. In the Co 2p spectra of pure Co₉S₈ and Ni-doped Co₉S₈ indicated the presence of Co²⁺ and Co³⁺ in the tetrahedral and octahedral sites. The peaks at ∼778.34 and 793.42 eV for Co₉S₈ were ascribed to Co³⁺, while those at 780.81 and 796.62 eV belonged to Co²⁺.³⁵ It can be seen from Fig. 4b that the Co³⁺ area of Ni-Co₉S₈-0.6 is higher than Co₉S₈, the ratio of Co³⁺/Co²⁺ obtained from their respective main lines is 0.3529 for Ni-Co₉S₈-0.6 and 0.3248 for Co₉S₈. The peaks located at 712.40 and 724.30 eV, with the satellite of 718.60 and 736.41 eV, indicated the existence of Ni³⁺, which caused the increase of Co³⁺/Co²⁺ ratio in Ni-Co₉S₈-0.6 microspheres. Meanwhile, the Ni³⁺/Ni²⁺ ratio of is 1.88 for Ni-Co₉S₈-0.3 and 2.38 for Ni-Co₉S₈-0.6 (Table S1†). Thus, we conclude that the nickel ions doping supply more trivalent sites in Co₉S₈ structure, which is helped to regulate cationic distribution and increase the output voltage of the electrode materials. The typical binding energy peaks S 2p_3/2 and S 2p_1/2 were located at 161.46 and 162.68 eV, respectively, the peak located at 59.60 eV may be ascribed to a small degree of surface oxidation of the catalyst exposed to the air condition.

Fig. 4 (a) XPS survey of Ni_xCo_9−xS₈ series hollow spheres. The high-resolution spectra of (b) Co 2p, (c) Ni 2p, and (d) S 2p.

3.1 Electrochemical performance of mesoporous Ni_xCo_9−xS₈ hollow microspheres

To evaluate the supercapacitance performance of the series Ni_xCo_9−xS₈ samples, electrochemical tests were conducted by a standard three-electrode system in 2.0 M KOH aqueous solution, with sample coated Ni foam as work electrode, Hg/Hg₂Cl₂ and Pt plate as reference electrode and counter electrode. The CV curves of the Co₉S₈, Ni-Co₉S₈-0.3, and Ni-Co₉S₈-0.6 electrodes were plotted in Fig. 5a, S3 and S4.† Co₉S₈ sample present two pairs of redox peaks in the CV curve, which can be attributed to the redox reaction of Co₉S₈ with two Co crystallographic sites in its structure. Compared with the Co²⁺/Co³⁺ and Co³⁺/Co⁴⁺ redox peaks of Co₉S₈ electrode, the Ni doped Co₉S₈ exhibited an obvious increase of peak intensity and increased integration area because of the coexistence of Ni²⁺/Ni³⁺, indicated the better performance of the Ni-doped samples. Fig. 5b indicated the intercalation and surface reaction mechanism occurred in the series of Ni_xCo_9−xS₈ electrodes. The corresponding diffusion contribution of Co₉S₈, Ni-Co₉S₈-0.3 and Ni-Co₉S₈-0.6 is 48%, 56%, and 61%, respectively, demonstrating the diffusion-controlled behavior at 5 mV s⁻¹. Conversely, with the increase of scan rate, the capacitive contribution enhanced due to the insufficient time for intercalation reaction into the series of Ni_xCo_9−xS₈ electrodes. The galvanostatic charge–discharge (GCD) plot of the three electrodes was shown in Fig. 5c and S5.† At a current density of 1 A g⁻¹, the Ni-Co₉S₈-0.6 electrode exhibits the longest discharge time, furthermore, according to the galvanostatic discharge curves, the calculated capacitances of Ni-Co₉S₈-0.6, Ni-Co₉S₈-0.3, and Co₉S₈ electrodes are 1300, 653, and 430 F g⁻¹, respectively. As the current density increase to 10 A g⁻¹, the corresponding capacitance decreased to 1048, 536, and 346 F g⁻¹ (Fig. 5d), and the corresponding capacitance retentions are 80.6%, 82.1%, and 80.5%, respectively. The above results indicated the introducing of Ni ions can effectively increase the specific capacitance and rate capability of pure Co₉S₈ sample. Fig. 5e showed the long-term cyclic performance of the three electrodes at 4 Ag⁻¹. The capacity of Ni-Co₉S₈-0.6 electrode attenuate to 74.5% after 5000 repeated cycles. More importantly, the mesoporous hollow Ni-Co₉S₈-0.6 electrode exhibited a performance comparable to or higher than the recently reported sulfide-related electrodes such as rGO/Co₉S₈ (575.9 F g⁻¹, 2 A g⁻¹),³⁶ CoNi₂S₄/Co₉S₈ composites (1183.3 F g⁻¹, 2 A g⁻¹),³⁷ Ni₃S₄@Co₉S₈ tubes (1002.2 F g⁻¹, 1 A g⁻¹),³⁸ CoO/Co₉S₈ hollow microspheres (1100 F g⁻¹, 2 A g⁻¹),³⁹ GH@NC@Co₉S₈ composite (540 F g⁻¹, 1 A g⁻¹),⁴⁰ Co₉S₈ nanowire/Ni (1191.17 F g⁻¹, 2 mV s⁻¹),⁴¹ 0.3 cP/rGO/Co₉S₈ (788.9 F g⁻¹, 1 A g⁻¹),⁴² Co₉S₈@N-C@MoS₂ (410 F g⁻¹, 10 A g⁻¹),⁴³ Co₃(OH)₂(HPO₄)₂ @Co₉S₈/Co electrode (1279.4 F g⁻¹, 5 mA cm⁻²),⁴⁴ CoS@NSC-800 (289 F g⁻¹, 1 A g⁻¹),⁴⁵ Co₉S₈@NiCo₂S₄@NF electrode (1026 F g⁻¹, 1 A g⁻¹),⁴⁶ (Co_0.94Fe_0.06)₉S₈ hollow spheres (454 F g⁻¹, 1 A g⁻¹),⁴⁷ Co₉S₈-2@CN/NF (471.1 F g⁻¹, 0.5 A g⁻¹),⁴⁸ Co₉S₈–aCNT–NiCoLDH (1185.5 F g⁻¹, 1 A g⁻¹),⁴⁹ and MnCo₂S₄/Co₉S₈/Ni composites (1058.0 F g⁻¹, 1 A g⁻¹).⁵⁰ A more extensive comparison of super-capacitance performance of Ni-Co₉S₈-0.6 with other related electrodes are listed in Table S2.† Furthermore, the improved electrochemical performance of Ni-Co₉S₈-0.6 can be supported by the smallest electrolyte resistance (R_s) and charge transfer resistance (R_ct) shown in Fig. 5f. The electrolyte resistance of Co₉S₈, Ni-Co₉S₈-0.3, and Ni-Co₉S₈-0.6 is 1.09, 1.29, and 0.97 Ω, respectively. Furthermore, the charge transfer resistance of Co₉S₈ is 3.39 Ω, which decreased to 1.39 Ω in Ni-Co₉S₈-0.6. The straight line with the largest slope in low frequency region also indicated the Ni-Co₉S₈-0.6 electrode exhibit the fast ionic diffusion efficiency and excellent reversibility (Fig. S6†).

Fig. 5 (a) Comparison of CV curves for Ni_xCo_9−xS₈ series hollow spheres at 5 mV s⁻¹, (b) the diffusion and capacitive-controlled contribution of Co₉S₈ (black), Ni-Co₉S₈-0.3 (blue), and Ni-Co₉S₈-0.6 (red), (c) GCD of Ni-Co₉S₈-0.6, (d) the rate capability of Ni_xCo_9−xS₈ series electrodes, (e) the cyclic stability at 4 A g⁻¹, and (f) Nyquist plots of Ni_xCo_9−xS₈ series electrodes.

Furthermore, a typical Ni-Co₉S₈-0.6//Zn alkaline battery is fabricated by using Ni-Co₉S₈-0.6 as cathode, Zn plate as anode, and 2 M KOH+0.2 M Zn(Ac)₂ as electrolytes. Fig. 6a shows the CV curves of Ni-Co₉S₈-0.6//Zn battery at the scan rate of 1–20 mV s⁻¹ with the potential range of 1.2–2.0 V vs. Zn. At 1 mV s⁻¹, the Ni-Co₉S₈-0.6//Zn displays a pair of redox peaks at 1.8 and 1.65 V. Even the scan rate up to 20 mV s⁻¹, the redox peaks can be preserved, indicated the excellent reversibility of the full battery. The GCD curves obtained at different current densities were shown in Fig. 6b, the GCD curves present a charge and discharge platforms located at 1.774 and 1.675 V at 1 A g⁻¹, with the voltage hysteresis of 0.099 V, demonstrating a less polarization and high energy conversion efficiency at the current density of 1 A g⁻¹. Furthermore, the specific capacitance of the Ni-Co₉S₈-0.6//Zn battery decreased from 152 to 97 mA h g⁻¹ with its voltage reduced from 1.675 to 1.654 V as the current density increasing from 1 to 10 A g⁻¹. The capacitance retention of 10 A g⁻¹ is 64%, indicating the excellent rate performance of Ni-Co₉S₈-0.6//Zn battery in alkaline electrolyte. Moreover, when the discharge current density came back to 1 A g⁻¹, the battery can restore the capacitance of 140 mA h g⁻¹ (92%), illustrating the tolerance to high speed charge–discharge reaction (Fig. 6c). The maximum charge capacitance approach to the similar aqueous Ni–Zn batteries, such as G-NCGs//Zn battery (113.8 mA h g⁻¹, 0.5 A g⁻¹),⁵¹ Ni₃S₂/Ni//Zn battery (148 mA h g⁻¹, 0.2 A g⁻¹),⁵² Zn//NiCo₂O₄ (112 mA h g⁻¹, 1 A g⁻¹),⁵³ Zn//NiO–CNTs battery (155 mA h g⁻¹, 1 A g⁻¹),⁵⁴ F-doped Ni–Co hydroxide (FNCH) and oxide constructed FNCH/Ni//Zn battery (88 mA h g⁻¹, 1 A g⁻¹) and FNCO/Ni//Zn battery (159 mA h g⁻¹, 1 A g⁻¹),⁵⁵ ZnCo₂O_4−x//Zn (148.3 mA h g⁻¹, 0.05 A g⁻¹),⁵⁶ Zn//ZnAl_0.67Co_1.33O₄ (134 mA h g⁻¹, 16 mA g⁻¹),⁵⁷ NiCo₂O₄-1//Zn (122.5 mA h g⁻¹, 1 A g⁻¹),⁵⁸ and NiO/C–Zn (159 mA h g⁻¹, 1 A g⁻¹).⁵⁹ Furthermore, the cyclic stability of the Ni-Co₉S₈-0.6//Zn battery was tested at 4 A g⁻¹ for 3000 cycles, the capacity retention kept at 69% (Fig. 6d). The excellent rate capability of Ni-Co₉S₈-0.6//Zn can be supported by the small charge transfer resistance (Fig. 6e). The power density and energy density of Ni-Co₉S₈-0.6//Zn battery was calculated and shown in Fig. 6f, the Ni-Co₉S₈-0.6//Zn battery can achieve the energy density of 256.5 W h kg⁻¹ (at 1.69 kW kg⁻¹) and the maximum power density of 16.56 kW kg⁻¹ (at 160.1 W h kg⁻¹). The power/energy density of Ni-Co₉S₈-0.6//Zn battery are comparable with the reported Ni–Zn battery such as β-Ni(OH)₂/CNFs//Zn (325 W h kg⁻¹ at a power density of 1.23 kW kg⁻¹),⁶⁰ Co₃O₄@NiO NSRAs//Zn (215.51 W h kg⁻¹ at 3.45 kW kg⁻¹),⁶¹ Co-Ni₃Se₂//Zn (199.34 W h kg⁻¹),⁶² P-Co₃O₄//Zn battery (193.7 W h kg⁻¹ at 1.6 kW kg⁻¹),⁶³ N-Fe₂O_3−x//Zn battery (135 W h kg⁻¹ at 0.47 kW kg⁻¹),⁶⁴ Am-NCS//Zn (254.2 Wh/kg at 3.28 kW kg⁻¹).⁶⁵ The excellent performance of Ni-Co₉S₈-0.6 should be ascribed to the unique mesoporous hollow spheres structure with enriched contact area and highly open structure, which facilities the mass transportation, and the Ni dopant in the Co₉S₈ matrix can improve the electrical conductivity.

Fig. 6 The electrochemical characterization of Ni-Co₉S₈-0.6//Zn battery (a) CV curves, (b) GCD curves, (c) rate capability, (d) the cyclic stability at 4 A g⁻¹, (e) Nyquist plot, and (f) Ragone plot.

4 Conclusions

In summary, a series of mesoporous hollow Ni_xCo_9−xS₈ spheres were synthesized via a facile solvothermal method, which exhibit excellent rate capability and long-time cycling performance. Attributed to the improvement of conductivity and abundant active surface, the capacitance of Ni-doped Co₉S₈ increased dramatically. Remarkably, the optimized Ni-Co₉S₈-0.6 electrode delivery a capacity of 1300 F g⁻¹ at 1 A g⁻¹. Especially, a stable aqueous Ni/Co–Zn battery was constructed by Ni-Co₉S₈-0.6 as cathode and Zn plate as anode. The Ni-Co₉S₈-0.6//Zn battery delivers an energy density of 256 W h kg⁻¹ and power density of 1.69 kW kg⁻¹. This work provides a facile method to prepare mesoporous hollow structure and cathodes for aqueous Ni–Zn batteries.

Author contributions

Daojun Zhang: methodlogy, writing and editing. Jingchao Zhang: analysis and investigation. Jiaqi Li and Yuting Li, synthesis and characterization. Chenxiang Li and Yingying Liu, software and data curation. Renchun Zhang_, editing and review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Science Foundation of China (No. 21603004), the Foundation of Henan Educational Committee (22A150002), and the Science and Technology Research Project of Henan Province (222102240096).

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Footnote

† Electronic supplementary information (ESI) available: the size distribution, EDX, CV curves, and GCD curves of Ni_xCo_9−xS₈ hollow microspheres. The XPS peak area ratio and comparison of the supercapacitor performance. See https://doi.org/10.1039/d2ra03022e

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