Novel hollow MoS₂@C@Cu₂S heterostructures for high zinc storage performance

Abstract

Heterostructured materials have great potential as cathodes for zinc-ion batteries (ZIBs) because of their fast Zn²⁺ transport channels. Herein, hollow MoS₂@C@Cu₂S heterostructures are innovatively constructed using a template-engaged method. The carbon layer improves the electrical conductivity, provides a high in situ growth area, and effectively restricts volume expansion during the recycling process. MoS₂ nanosheets are grown on the surfaces of hollow C@Cu₂S nanocubes using the in situ template method, further expanding the specific surface area and exposing more active sites to enhance the electrical conductivity. As expected, an admirable reversible capacity of 197.2 mA h g⁻¹ can be maintained after 1000 cycles with a coulombic efficiency of 91.1%. Therefore, we firmly believe that this work points the way forward for high-performance materials design and energy storage systems.

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Ren Cai

Dr. Ren Cai, a Professor at Hunan University, earned his Ph.D. in Analytical Chemistry from the University of Florida (USA) in 2017. Dr. Cai's research interests focus on novel analytical technologies, nanomedicine, and novel biosensors. At present, Dr. Cai has published over 70 peer-reviewed articles, including over 50 articles as the first author or corresponding author at J. Am. Chem. Soc., Anal. Chem., et al. Dr. Cai serves as the young editorial board of Collagen and Leather and Exploration, respectively.

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Introduction

With the development of advanced energy storage technologies, lithium-ion batteries (LIBs) have exhibited some inevitable defects, such as high cost and potential safety hazards.^1–3 Currently, aqueous system zinc ion batteries (ZIBs) are expected to be alternatives to LIBs due to their low cost, abundant resources, and high security.^4,5 Although there have been many advances in the research on ZIBs, the design of high-performance ZIBs still faces great challenges, such as slow diffusion of zinc ions and low reversibility, which seriously hinder their development and commercialization.^6,7

As a new type of 2D structure, many transition metal dichalcogenides (TMDs) are attracting attention as novel energy storage materials, because of their efficient energy conversion.^8–11 For example, VS₄@rGO nanosheets were explored using a hydrothermal process for super-performance ZIBs;¹² as a novel electrode, WS₂ nanosheets exhibited high specific capacity and were applied in sodium-ion batteries;¹³ SnS₂/Ti₃C₂T_x nanosheets were fabricated using an in situ hybridization method for high-performance potassium-ion batteries.¹⁴ More interestingly, CuS nanocubes exhibited high energy density when used as cathode materials,¹⁵ and MoS₂ nanosheets are conducive to ion intercalation/deintercalation in lithium/sodium-ion batteries.^16,17 However, there are some challenges in the application of MoS₂ nanosheets in ZIBs, such as poor conductivity, higher hydrophilicity, agglomeration, and volume expansion.^18–20 Therefore, it is important to explore novel MoS₂ structures with high conductivity and cycling stability for energy storage.

Herein, we fabricated novel MoS₂@C@Cu₂S heterostructures using a template-engaged process for high-capacity Zn²⁺ storage (Scheme 1). These heterostructures are composed of MoS₂ nanosheets and hollow C@Cu₂S nanocubes, and exhibit high specific surface area, exposing many active sites and enhancing the electrical conductivity in ZIBs. The hollow cavity facilitates rapid Zn²⁺ transport to reduce the energy barrier of Zn²⁺ absorption and transport, and reinforces the volume elasticity to accommodate volume change during Zn²⁺ de-/intercalation. At current densities of 0.1 A g⁻¹ and 1.0 A g⁻¹, the as-designed ZIBs exhibit stable specific capacities of 679.1 mA h g⁻¹ and 192.9 mA h g⁻¹, respectively.

Scheme 1 Synthetic process of the hollow MoS₂@C@Cu₂S heterostructures.

Results and discussion

The hollow MoS₂@C@Cu₂S heterostructures (size of ∼600 nm) were prepared using an etching process followed by hydrothermal reduction (Scheme 1 and Fig. 1a and b). First, hollow CuS nanocubes with uniform sizes (size of ∼500 nm, Fig. S1b†) were prepared by the reaction of Cu₂O nanocubes (size of ∼550 nm, Fig. S1a†), Na₂S, and HCl. After carbonization, PDA@CuS nanocubes converted into hollow C@Cu₂S nanocubes (size of ∼550 nm, Fig. S1c†). Finally, hollow heterostructures were prepared after folding MoS₂ nanosheets were grown on the surface of hollow C@Cu₂S nanocubes (Fig. 1c).

Fig. 1 Characterization of MoS₂@C@Cu₂S heterostructures: (a) SEM image; (b–d) TEM images; (e) elemental mapping images.

The X-ray powder diffraction (XRD) pattern shows that all peaks of the crystal phases are indexed to MoS₂ (JCPDS: 37-1492) and Cu₂S (JCPDS: 72-2276) (Fig. S2†). As exemplified in elemental distribution scanning (Fig. 1e), the distribution of C can be seen in the intermediate layer (Fig. S3†). These hollow CuS nanocubes with folding MoS₂ nanosheets and the C layer are MoS₂@C@Cu₂S heterostructures. As further evidence, the lattice diffraction fringe at 0.606 nm corresponds to the (0 0 2) crystal plane of MoS₂, and the fringe at 0.260 nm corresponds to the (1 0 3) crystal plane of Cu₂S in the high-resolution TEM image (Fig. 1d). X-ray photoelectron spectroscopy (XPS) further confirmed that the hollow heterostructure is composed of S, Mo, C, and Cu (Fig. S3†). The E¹_2g and A_1g peaks from MoS₂, the Cu–S vibration, and the D-band (defect vibration) and the G-band (graphitic carbon vibration) from C are observed in Raman spectroscopy (Fig. S4†), demonstrating the presence of MoS₂, CuS and the carbon layer in the heterostructures. In addition, the heterostructures show typical meso-shell structures with a large specific surface area of 34.10 m² g⁻¹ (Fig. S5†).

The electrochemical properties of the MoS₂@C@Cu₂S heterostructures were investigated as shown in Fig. 2. As shown in Fig. 2a, the irreversible cathodic peaks at 0.32 V result from multiple electrochemical reactions, which involve the insertion of Zn²⁺ to form Zn_xCu₂S/Zn_xMoS₂ and to generate ZnS and Mo. In the subsequent positive scan, the peak at 0.71 V corresponds to the dezincing of ZnS to re-generate Cu₂S and MoS₂. Due to the formation of a solid electrolyte layer (SEI) film during the initial activation process, the loss of irreversible capacity results in a larger area in the initial cycle than that in the following cycles. From the second cycle to the fifth cycle, the curves show good overlap, indicating good cycling stability.

Fig. 2 The electrochemical performance tests of MoS₂@C@Cu₂S heterostructures: (a) CV profiles at 0.1 mV s⁻¹; (b) charge–discharge curves of the first, fifth and tenth cycles; (c) rate performance; (d) long-term cycling performance; (e) high current density performance.

In Fig. 2b, the galvanostatic charge–discharge (GCD) test exhibits the charge–discharge curves of the heterostructures at a current density of 0.1 A g⁻¹. The charge and discharge curves of the first, fifth, and tenth cycles show a high degree of similarity, indicating good cycling stability. For the insertion of Zn²⁺, the voltage platforms are formed in the discharge process at 0.32 V. At 0.71 V, the platform is in the charging state, corresponding to the dezincing process. These results indicate good agreement between the GCD test and the CV curve (Fig. 2a and b). At different current densities of 0.1, 0.2, 0.5, 0.8, 1.0, 0.8, 0.5, 0.2, and 0.1 A g⁻¹, the corresponding specific discharge capacities are 548.6, 420.2, 345.2, 292.2, 248.9, 304.8, 349.1, 424.3, and 534.1 mA h g⁻¹, respectively (Fig. 2c).

The specific capacities of the heterostructures regularly decrease with increasing current density. In contrast, when the current density decreases, the specific capacity increases, proving the excellent rate performance. Fig. 2d shows the cycling performance and the corresponding coulombic efficiency of the heterostructures after 1000 cycles at 0.1 A g⁻¹. The initial specific capacity is 679.1 mA h g⁻¹, and the specific capacities during the first few cycles are gradually enhanced, which is related to the activation of the heterostructures.²² After 1000 cycles, the specific capacity is 197.2 mA h g⁻¹ and the coulombic efficiency is 91.1%. Furthermore, at a high current density, i.e., 1 A g⁻¹, the specific capacity is 192.9 mA h g⁻¹ after 300 cycles. These results confirm that the MoS₂@C@Cu₂S heterostructures have excellent cycling stability for ZIBs at high current density. In order to investigate the reaction mechanism of Zn²⁺, the electrochemical characteristics of MoS₂@C@Cu₂S heterostructures in an aqueous electrolyte were studied using CV in the potential range of 0.2–1.0 V (vs. Zn²⁺/Zn). In Fig. 3a, a pair of redox peaks at 0.25/0.75 V is observed, representing the reversible de-intercalation process of Zn²⁺ in MoS₂ nanosheets. There is no change in the shape of the CV curves when the scanning rate increases from 0.1 to 1.0 mV s⁻¹, which indicates that the heterostructures possess good electrochemical reversibility. With an increased scan rate, the anodic peak moves to a high potential (i.e., 0.76 V) from 0.72 V, and the cathodic peak shifts to a low potential (i.e., 0.25 V) from 0.38 V. These potential changes are related to the enhanced diffusion resistance.

Fig. 3 (a) CV profiles at different scan rates; (b) plot of log (sweep rate) versus log (peak current) for anodic and cathodic peaks; (c) the capacitive contribution to the total capacity at 0.6 mV s⁻¹; (d) capacity contribution at different scan rates; (e) EIS plots of MoS₂@C@Cu₂S heterostructures before and after 100 cycles; (f) GITT test conditions for the heterostructures during the discharge process; (g) GITT profiles and the calculated D_Zn²⁺ values; (h) comparison of the rate performance with those of previously reported MoS₂-based electrode materials applied in ZIBs.

The electrochemical kinetics of the reduction of Zn²⁺ are presented in Fig. 3b. The power formula shows that I = aν^b, log(i) = log(a) + b log(v), where I is the current, ν is the scanning rate, and a and b are adjustable parameters. The coefficient b represents the type of electrochemical charge storage. When b is in the range of 0.5 to 1, the capacitance controls the process of charge storage.²¹ Here, b was calculated to be 0.78 and 0.69 for the oxidation peak and the reduction peak, respectively, indicating that the Zn²⁺ storage process is controlled by a synergistic mechanism between the capacitive contribution and the diffusion process. The contributions of the capacitive effect (k₁ν) and diffusion effect (k₂ν^1/2) were further calculated according to the formula: i = k₁ν + k₂ν^1/2. The contribution ratio of the capacitance to the diffusion capacity of the heterostructures was calculated to be ∼50.41% at a scan rate of 0.6 mV s⁻¹ (Fig. 3c). In Fig. 3d, with the increasing scan rate, i.e., 0.2 to 1.0 mV s⁻¹, the capacitance contribution is enhanced from 49% to 57%, indicating that a capacitive-controlled process dominates the storage behavior to promote the rate performance and capacity performance of ZIBs.

Next, electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration technique (GITT) tests were executed to evaluate the kinetics of the electrode reaction in detail (Fig. 3e and f). There is a small semicircle diameter in the EIS curves of MoS₂@C@Cu₂S heterostructures before and after the cycle (Fig. 3e), indicating that the heterostructures possess good electrical conductivity and rapid Zn²⁺ kinetic diffusion. Based on Fick's second law, the difference in Zn²⁺ diffusion kinetics is explored as follows:

where τ, m_B, M_B, V_M, and S stand for the current pulse time (s), the mass, the molar mass, the molar volume of active materials, and the area of the electrode, respectively;²³ the calculation method of ΔE_S and ΔE_τ is also displayed in Fig. 3f. It is clearly seen that the pulse time is 5 min, the relaxation time is 10 min, and the quasi-open circuit voltage is reached, and by calculation the D_Zn²⁺ was obtained in the range from 10⁻⁸ to 10⁻¹⁰ cm² s⁻¹ (Fig. 3g), indicating that the electrode possesses good reaction kinetics.^24,25 Compared with previous reports on MoS₂-based zinc-ion batteries (Fig. 3h),^26–28 MoS₂@C@Cu₂S heterostructures exhibit high performance for ZIBs at different current densities. The excellent electrochemical characteristics of the heterostructures are closely related to the following factors: (1) ultra-thin MoS₂ nanosheets cover the surface of hollow C@Cu₂S nanocubes, which would promote fast ion transport and effectively alleviate the volume expansion;²⁹ (2) the hollow cavity, the high specific surface area (i.e., 34.10 m² g⁻¹), and a large number of electrochemical active sites would shorten the ion diffusion path;^19,25,30 (3) the intermediate C layers maintain the stability of the heterostructures.²⁶ All three factors enable MoS₂@C@Cu₂S heterostructures to achieve superior reversible cycling performance. Density functional theory (DFT) calculations were used to analyze the diffusion of Zn²⁺ inserted in the lattice of MoS₂@C@Cu₂S heterostructures at different energy barriers. Fig. 4a exhibits the theoretical laminated structure, and Fig. 4b shows the heterostructure of the interlayer model of Zn²⁺ insertion. Moreover, compared with the Zn²⁺ diffusion barriers of pure MoS₂ (Fig. 4c and d), the interlayer diffusion energy barrier of Zn²⁺ decreases in MoS₂@C@Cu₂S heterostructures after MoS₂ nanosheets grow on the surface of hollow C@Cu₂S nanocubes, which further indicates that the heterostructures are conducive to Zn²⁺ diffusion.

Fig. 4 (a) Optimized structure of MoS₂@C@Cu₂S; (b) optimized structure of Zn²⁺ insertion in MoS₂@C@Cu₂S; (c) top view of the optimized structure of Zn²⁺ insertion in pure MoS₂ and MoS₂@C@Cu₂S; (d) energy barriers of diffusion behavior of Zn²⁺ in pure MoS₂ and MoS₂@C@Cu₂S.

Conclusions

In summary, novel hollow MoS₂@C@Cu₂S heterostructures were fabricated using a template-engaged process for Zn²⁺ storage. These heterostructures with sizes of ∼600 nm are packed by folding MoS₂ nanosheets and hollow C@Cu₂S nanocubes. Because of the high surface area, i.e., 34.10 m² g⁻¹, these heterostructures exhibit a high initial specific capacity of 679.1 mA h g⁻¹ and a reversible capacity of 197.2 mA h g⁻¹ with a coulombic efficiency of 91.1% after 1000 cycles at 0.1 A g⁻¹. When the capacity was tested at 1.0 A g⁻¹, the structure delivered 192.9 mA h g⁻¹ after 300 cycles. Furthermore, the hollow heterostructures exhibit excellent rate performance. Therefore, this work provides an effective design of heterogeneous materials for applications in power grid energy storage systems using ZIBs in the future.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge the financial support from the Talent Introduction Start-Up Foundation of Guangxi Minzu University (2021KJQD08), the Natural Science Foundation of China (No. 22074130), the Key Scientific Research Projects of Henan Province (232102320040), the National Natural Science Foundation of China (22004032) and the Hunan Provincial Natural Science Foundation of China (2021JJ20020). The authors also acknowledge the great support from the Analysis Testing Center of Xinyang Normal University for materials characterization.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3nr05231a

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