0 Introduction

In the past several decades, lithium-ion batteries (LIBs) have become part of our daily life in many aspects ^[1-2]. However, LIB's resources, cost, safety and a slew of other issues will stymie their development in the future. Compared with LIBs, aqueous rechargeable batteries have many advantages including excellent safety, affordability, environmental friendliness and high ionic conductivity, making them ideal for energy storage field ^[3-8].As an important part of secondary batteries, electrode materials deserve our attention ^[9-12]. However, the aquatic environment's complexity has a great impact on battery performance. Thus, an electrode material that can work in various environments has great potential.

Copper sulfide (CuS) is a 2D material with excellent physical and chemical properties that is frequently employed in the disciplines of light, electricity, magnetism, lubrication and catalysis^[13]. Due to its excellent electrical, optical and other outstanding electromagnetic properties, nanostructured CuS has garnered a lot of attention as a key component in sulfur metal complexes^[14-15]. CuS and cuprous sulfide (Cu₂S) nanostructures with various morphologies have been created, including nanoparticles, nanoplatelets, nanorods, nanoribbons, nanoflowers, and hollow nanospheres. It features unique nanostructures that provide it a satisfactory specific surface area. This, along with its excellent electric conductivity, allows it to hold more charge capacity and makes it a desirable electrode material^[16-18].

CuS electrode materials with varied morphologies have recently been produced using various synthesis methods, and their many applications have been investigated. Zhang et al.^[19] designed a CuS/ cetyltrimethylammonium bromide superlattice, which is applied to anode for Zn ion battery with excellent rate performance (225.5 mA·h/g at 1 A/g and 140 mA·h/g at 10 A/g). CuS porous nanocages were produced by a one-step liquid-phase technique^[20].The unique nanostructure enhances ion transport, which provides a high specific capacity up to 228 mA·h/g(take Mg ion as an example). Wang et al.^[21] reported CuS nanosheet arrays, which exhibited outstanding electrochemical performance. The CuS nanosheet arrays show a large capacity of 510 mA·h/g and ultra-stable cycling (91% of the initial capacity over 2500 cycles).

In this work, a CuS electrode with layer flake nanostructures that can work at various pH is presented, which can carry and store non-metallic ions and metallic ions, including multivalent ions with high charge radius ratio. Its electrochemical behavior is evaluated in acidic, neutral and alkaline environments.CuS//PbO₂ is successfully assembled with H₂SO₄ electrolyte, exhibiting a considerable capacity of 240 mA·h/g. In neutral electrolytes, CuS can operate as stable electrode materials with different charge carrier species. In addition, in alkaline KOH electrolyte, the Zn//CuS batteries present a considerable capacity of ~ 900 mA·h/g at 0.5 A/g. Additionally, the battery also has a long discharge platform at 1 A/g which contributes to more than 80% of the total capacity. Even as the current density is raised, the ultra-long discharge platform remains.

1 Experimental Section 1.1 Material Preparation

First, 2 mmol Cu (CO₂CH₃)₂·H₂O, 2 mmol NaOH, 1 mmol urea (CH₄N₂O) and 60 mL deionized water (DI) were stirred at room temperature for 30 min. After that, the resulting product was poured into a polytetrafluoroethylene reactor and hydrothermally treated for 24 h at 120 ℃. The resultant powder was washed 3 times with DI and ethanol, respectively. Finally, the prepared powder and sulfur powder (purity 99%) were heated for 3 h at 350 ℃ in atube furnace under Ar gas.

1.2 Material Characterizations

The X-ray diffraction (XRD) patterns and Raman spectrum of CuS were examined by a D8 super speed diffractometer (Bruker AXS, Germany) and micro-Raman spectroscopy system (532 nm laser excitation, Renishaw INVIA). The morphologies of the CuS layer flake nanostructures were examined using scanning electron microscope (SEM) (Hitachi, Japan). The chemistry of the sample was characterized by X-ray photoelectron (XPS) using Thermo Fisher Scientific Escalab 250Xi. The surface area of CuS power was investigated by nitrogen adsorption-desorption isotherms at 77 K (Tristar II 3020). The high-resolution image and mapping were recorded by transmission electron microscopy (TEM) (JEM-2100F).

1.3 Electrochemical Characterizations

The CuS electrode was synthesized by mixing slurry composed of 80 wt % of CuS, 10 wt % ketjen black, and 10 wt % PVDF for 15 min. Then, N-methyl pyrrolidone (NMP) was added and stirred for 15 min. The slurry was casted onto carbon cloth and dried at 60 ℃ for 12 h.

The acid electrolyte is 1 mol/L H₂SO₄. CuS (working electrode), Pt (counter electrode) and Ag/AgCl (reference electrode)were assembled into a three-electrode system to evaluate the H⁺ storage performance of CuS. Meanwhile, CuS//PbO₂ was assembled and tested in 1 mol/L H₂SO₄. NH₄⁺, Li⁺, Na⁺, K⁺, Ca²⁺, Mg²⁺, Zn²⁺ and Al³⁺ storage performance were also evaluated by above system except different electrolyte. An aqueous solution of (NH₄)₂SO_4, Li₂SO₄, Na₂SO₄, K₂SO₄, CaCl₂, MgSO₄, ZnSO₄, or Al₂(SO₄)₃ (concentrations all were 1 mol/L) was used for NH₄⁺, Li⁺, Na⁺, K⁺, Ca²⁺, Mg²⁺, Zn²⁺ and Al^{3 +} storage, respectively. For alkaline electrolyte, CuS and Zn were used as electrodes and 2 mol/L KOH + 0.02 mol/L Zn (Ac)₂ as electrolyte, so that the Zn//CuS batteries were assembled and tested.

The galvanostatic charge/discharge (GCD) data were recorded with LAND CT2001A. Cyclic voltammetry (CV) was recorded using a CHI760E electrochemical workstation.

2 Results and Discussion

The CuS nanosheets were prepared using a hydrothermal and vulcanization treatment. The crystal structure of samples was examined by XRD technology. As shown in Fig. 1(a), the standard CuS diffraction peak of pure hexagonal phase corresponds to all XRD peaks, and apparent diffraction peaks arise at the diffraction angle, indicating that the crystal plane is consistent with the standard card characteristic peak (JCPDS: 06-0464). The better the crystallinity of CuS, the sharper the peak. There is no further imperfection visible, implying that the product is pure hexagonal. Raman test (Fig. S1(a)) results show that there are two obvious peaks at 264 and 472 cm^-1, which correspond to the standard covellite CuS. Therefore, the above results further prove that the synthesized material is CuS. Then, the N₂ adsorption/desorption isotherms (Fig. S1(b)) illustrate the specific surface area is 2.0 m²/g, along with a mesopores structure. In order to further prove the structure of CuS, the valence state of the synthesized powder was analyzed by X-ray photoelectron spectroscopy (XPS). Fig. S1(c) shows two peaks at 932.0 and 951.9 eV belong to Cu²⁺ 2p_3/2 and Cu²⁺ 2p_1/2. In the S 2p spectrum (Fig. S1(d)), the binding energies of two peaks are 162.1 and 163.1 eV, deriving from S 2p_3/2 and S 2p_1/2 of S^2-. The high-resolution TEM image of CuS is shown in Fig. S2. It reveals the obvious lattice stripes with a plane spacing of 0.33 nm, correlating to (100) plane of CuS. An elemental mapping analysis reveals main elements are evenly distributed. The microstructure of CuS material was investigated via SEM (Fig. 1(b)). Obviously, the length of flake is about 1 μm which embeds ions well between layers, enhancing its electrochemical characteristics.Therefore, the electrochemical performance of CuS deserves further study.

Fig. 2 demonstrates the electrochemical performan-ces of CuS electrode in the acidic electrolyte. According to the CV curves at 5 mV/s (Fig. 2(a)), there are two prominent cathodic peaks (vs. Ag/AgCl), one at about -0.2 V, representing the embedding and reduction of Cu²⁺ to Cu⁺, and the other at about 0.2 V, representing the extraction of Cu^{+ [21]}. From the 1st to the 5th cycle, both the peak current and the area of CV curves decline somewhat, indicating a capacity reduction. Fig. 2(b) further shows the cycling performance of the CuS electrode in acidic electrolyte. The specific capacity is up to 80 mA·h/g at 0.5 A/g, then decays to 60 mA·h/g after 10 cycles, and thereafter tends to remain steady. Its coulombic efficiency is unstable, which is mainly attributed to the activation of electrode materials. During the activation process, the electrode material is still in an unstable state. With the increase of circulation, the electrode material continuously adsorbs/releases electrons to make it stable, which enhance the reversibility of the electroche-mical reaction^[22-23]. Furthermore, we attempted to utilize the CuS as the anode of the acidic cell due to its low redox potential. When coupled with a PbO₂ cathode, the battery displays an oxidation peak at about 1.5 V and a reduction peak at about 1.2 V (Fig. 2(c)) in the CV curve of 5 mV/s, which are comparable to the AC//PbO₂ hybrid battery (1.3 V)^[24]. The high discharge platform is also proved by the GCD curve of 0.5 A/g(Fig. 2(d)), which shows a considerable capacity of 240 mA·h/g. The average load of CuS electrode is 1.6 mg and the active mass ratio between CuS and PbO₂ for the CuS//PbO₂ battery is 0.5.

Furthermore, the electrochemical performance of the CuS electrode was explored in neutral electrolytes (Fig. 3). It is found that the CuS electrode can store and transport both non-metallic (NH₄⁺) and metal ions including monovalent Li⁺, Na⁺ and K⁺, and even multivalent Mg²⁺, Zn²⁺, Ca²⁺ and Al³⁺. All the reduction peaks of the CV curves are single peaks, indicating that the CuS is extremely easy to be reduced to Cu in the environment of neutral electrolyte. The CV curves exhibit good cycle stability when using 1 mol/L (NH₄)₂SO₄ as a non-metallic charge carrier at 5 mV/s(Fig. 3(a)). With the Li⁺, Na⁺ and K⁺ as charge carrier (Figs. 3(b)-(d)), it can be seen that the cycle stability is highest in the presence of Na⁺, and the reduction peak in K⁺ is the least visible, which could be ascribed to the large ionic radius of K⁺. Besides, the satisfactory redox process of CuS electrode was also demonstrated in the presence of multivalent ions such as divalent Ca²⁺, Mg²⁺, Zn²⁺ and even trivalent Al³⁺ (Figs. 3(e)-(f)). Different from acid electrolytes, the CV curve of CuS electrode shows only one reduction peak in neutral electrolytes. This result indicates that the reaction is partially irreversible, which corresponds well with the reported results^{[20, 25]}. It is mainly because of the fact that the valence states and radii of different ions are significantly different when the CuS cathode material is embedded and removed.

Fig. 4 further illustrates the cycling performance and GCD curves of the CuS electrode in a neutral electrolyte. In 1 mol/L (NH₄)₂SO₄ electrolyte, the discharging platforms at 0.3 V and 0.25 V can be clearly observed at 1 A/g (Fig. S3(a)). Astonishingly, the capacity remains stable, with a capacity of 100 mA·h/g after 1600 GCD cycles at 2 A/g (Fig. 4(a)).

As a typical representative of monovalent metal ions, the cycle performance of the CuS electrode in Li⁺ electrolyte displays a decreasing trend followed by an increasing trend. Specifically, the capacity decreases from 100 mA·h/g to 50 mA·h/g and then gradually increases to 200 mA·h/g after 12000 cycles (Figs. S3(b)-(c)).

When 1 mol/L CaCl₂ is used as electrolyte, the discharge capacity is 120 mA·h/g (Fig. 4(b)). At the condition of 0.35 A/g, the capacity is still up to 80 mA·h/g after 120 cycles (Fig. S3(d)). Low current density and high current density tests are carried out in MgSO₄ electrolyte. After 1000 cycles at a low current density of 0.22 A/g, the capacity drops from 80 mA·h/g to 40 mA·h/g. However, its coulomb efficiency is still close to 100% (Fig. 4(c)). During the GCD process of magnesium ions, there is a magnesium ion discharge platform of 0.2 V in the discharge curve. When the voltage drops to -0.4 V, hydrogen evolution reaction occurs, another platform appears (Fig. S3(e)). The capacity drops to 20 mA·h/g when the current density is amplified by 10 times (Fig. S3(f)). The SEM proves the stable structure of the CuS electrode after cycles (Fig. S4). CuS is still in the shape of nanosheets, which indicates the electrochemical stability in neutral environment. As for higher valence Al³⁺, there are two discharge platforms at 0.25 V and -0.3 V, respectively (Fig. 4(d)), it correspond to the two oxidation peaks in CV curves (Fig. 3(h)).

CuS electrode is further expanded in alkaline electrolytes. Herein, Zn is used as anode and 2 mol/L KOH + 0.02 mol/L Zn (Ac)₂ as electrolyte. The possible electrode reactions are proposed as following equations^[26]:

Positive electrode:

$ \begin{gathered} \mathrm{CuSOH}+\mathrm{e}^{-} \leftrightarrow \mathrm{CuS}+\mathrm{OH}^{-} \\ \mathrm{CuSO}+\mathrm{H}_2 \mathrm{O}+\mathrm{e}^{-} \leftrightarrow \mathrm{CuSOH}+\mathrm{OH}^{-} \end{gathered} $

Negative electrode:

$ \mathrm{Zn}+4(\mathrm{OH})^{-} \leftrightarrow \mathrm{Zn}(\mathrm{OH})_4{ }^{2-}+2 \mathrm{e}^{-} $

The Zn//CuS battery has a capacity of about 600 mA·h/g at 1 A/g, and retains at 200 mA·h/g after 50 cycles (Fig. 5(a)). A voltage window of 1.8 V can be observed on the CV curves (Fig.S5). Additionally, the Zn//CuS batteries show an extremely high capacity at different current densities when compared with other common cathode materials of Zn ion batteries (Fig. 5(b)). Notably, the Zn//CuS batteries have an ultra-long discharge plateau at 1 A/g, with the discharge plateau contributing more than 80% of the overall capacity (Fig. 5(c)). Fig.S6 shows the cycling performance and GCD curve at different current densities. Therefore, the plateau contribution decreases as the current density increases, but the ultra-long discharge plateau still exists, and its plateau voltage also appears to decline (Fig. 5(d)). Besides, the long cyclic stability at 5.0 A/g was evaluated in Fig. 5(e) and Fig.S7, which displayed high discharge capacity (200 mA·h/g after 200 cycles), along with ~100% coulombic efficiency. After electroche-mical cycle test, CuS polymerizes into flower shape, but the main body is still in the shape of nanosheets (Fig.S8). Obviously, there is a ultra-long discharge plateau, which maybe ascribe to the oxidation of S^2-[27-28].

3 Conclusions

In conclusion, CuS electrode can work at various pH, which can carry and store both non-metallic and metallic ions, including multivalent ions with high charge radius ratio. This also proves that CuS electrode can be used as a stable electrode material. The CuS//PbO₂ shows a high capacity of 240 mA·h/g in H₂SO₄ electrolyte. In addition, CuS can be designed as electrode with different charge carrier species (i.e., NH₄⁺, Li⁺, Na⁺, K⁺, Ca²⁺, Mg²⁺, Zn²⁺ and Al³⁺). In alkaline KOH electrolyte, the Zn//CuS batteries displays a considerable specific capacity (900 mA·h/g) and the ultra-long discharge platform. At the same time, the high-performance CuS electrode has practical application prospect because of its simple synthesis, pollution-free and inexpensive merits.

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