Superior-performance lithium–sulfur batteries: a face-centered-cubic-structure high-entropy alloy improves the bidirectional catalytic conversion of polysulfides/sulfides

Abstract

Herein, we demonstrate a face-centered cubic-structure cobalt–nickel–copper–manganese–molybdenum high-entropy alloy (CoNiCuMnMo-HEA) anchored on a reduced graphene oxide (rGO) substrate as a separator intercalation layer (CoNiCuMnMo-HEA@rGO) for lithium–sulfur batteries. The CoNiCuMnMo-HEA catalyst possesses abundant exposed polymetallic sites and exhibits a strong interaction with polysulfides on the (111) densest stacking surface. Experiments and theoretical calculations prove that Mn and Mo elements play a key role in the reduction process of polysulfides in CoNiCuMnMo-HEA, while Ni and Co are very important in the oxidation process of sulfides. Meanwhile, the presence of Cu element effectively regulates the redox process, which results in HEA exhibiting excellent bidirectional catalysis. As a result, the initial specific discharge capacity of a Li–S battery with the CoNiCuMnMo-HEA@rGO modified membrane can reach 1512.4 mA h g⁻¹ at a rate of 0.2 C. The attenuation rate of each cycle is only 0.055% after 1100 cycles at 3 C, and the coulombic efficiency remains above 91.94%.

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Introduction

The lithium–sulfur (Li–S) battery has the advantages of a high theoretical specific capacity and energy density, natural harmlessness and abundant resources and is expected to replace most of the current operating scenarios of lithium-ion batteries and alleviate the anxiety about the cruising range.^1–4 However, it is still difficult to commercialize Li–S batteries on a large scale. It mainly involves solid–liquid–solid conversion, such as cyclic S₈ in the solid phase is converted into liquid Li₂S_n (4 ≤ n ≤ 8), and finally into the solid product Li₂S₂/Li₂S, which leads to slow kinetics of the sulfur reduction reaction (SRR).^5–7 Meanwhile, the solution of liquid Li₂S_n in the electrolyte easily causes the shuttle effect and can induce dendrite growth and dead lithium problems in lithium anodes.^8,9 Based on this, how to quickly improve the SRR reaction is the key to inhibit the shuttle effect of polysulfides and the dendrite growth and lithium death of lithium anodes.

In order to solve the above problems, many researchers mainly build multi-level structures and create physical or chemical adsorption sites based on carbon-based materials to improve the utilization rate of sulfur and inhibit the shuttle effect of lithium polysulfides (LiPSs).¹⁰ Although these strategies can suppress the problems caused by LiPSs, the reaction kinetics of polysulfides has not been improved; therefore, the stability of the Li–S battery cannot meet the practical requirements.¹¹ Moreover, carbon-based materials loaded with too many metal compounds will significantly reduce the specific surface area and pore volume, thus reducing the sulfur loading.^12,13 Fortunately, carbon-based catalytic intercalation on the battery separator not only accelerates the SRR, but also does not affect the sulfur storage at the cathode, which is an ideal strategy.^12,14–16 Particularly, intercalation with mono/multi-metallic nanoalloys as catalysts has been extensively studied for Li–S batteries.^17,18 For example, Gao et al.¹⁸ designed a concave nano-cubic Ni–Pt alloy with high exponential surface constraint, which was introduced as a catalyst for Li–S batteries, significantly promoting the transformation of intermediate LiPSs into solid discharge products. However, due to the limited number of metal active centers, the conversion efficiency of solid sulfur into intermediate LiPSs is low, and it is difficult to achieve a long cycle life.¹⁹ Recently, high-entropy alloys formed from various metals have attracted considerable attention due to their high entropy.²⁰ They have many active sites such as lattice distortion effect, cocktail effect and slow diffusion effect and may have good catalytic activity.^21–23 For example, Wang et al.²⁴ designed and constructed Fe_0.24Co_0.26Ni_0.10Cu_0.15Mn_0.25 HEA as the core catalyst which has high electrocatalytic activity for the transformation of solid sulfur into solid discharge products. However, the reverse reaction to polysulfides was not significantly improved. To this end, the development of HEA catalysts with high activity for both bi-directional reactions of polysulfides/sulfides is particularly necessary for the commercial application of Li–S batteries.

Herein, we grow five-membered organometallic framework compounds (CoNiCuMnMo-MOF) on graphene oxide (GO) by a hydrothermal method. Subsequently, the CoNiCuMnMo-HEA nanoparticle-anchored reduced graphene oxide material (rGO) was successfully obtained after pyrolytic reduction (CoNiCuMnMo-HEA@rGO). As a catalytic intercalation material for the Li–S battery separator, it shows excellent performance. This is mainly due to the fact that the diameter of CoNiCuMnMo-HEA is about 12 nm, and it is dispersed in rGO with a high density to avoid agglomeration and form more catalytically active sites. In addition, CoNiCuMnMo-HEA possesses a face-centered cubic structure, and the exposed (111) close-packed surface shows good structural stability. The electrochemical test results show that CoNiCuMnMo-HEA@rGO exhibits a strong adsorption capacity for LiPSs, effectively accelerates the movement and diffusion of Li⁺, and at the same time has the catalytic advantage of effectively accelerating the conversion of Li₂S_n (4 ≤ n ≤ 8) into Li₂S₂/Li₂S. Furthermore, CoNiCuMnMo-HEA@rGO could accelerate the reverse catalytic conversion of Li₂S into liquid-phase LiPSs, realizing the bidirectional catalytic conversion of LiPSs. Furthermore, density functional theory (DFT) calculations reveal the unique electron co-regulation properties of CoNiCuMnMo-HEA, where HEA exhibits a lower Fermi energy level than any individual metal, which is more favorable for electron movement and transfer. With these advantages, the specific capacity of CoNiCuMnMo-HEA@rGO can reach 1512.4 mA h g⁻¹ at initial discharge and 570.2 mA h g⁻¹ at 5 C, and the specific capacity decay rate is only 0.055% after that per cycle for 1100 cycles at 3 C. The specific capacity efficiency remains above 91.94%. The efficiency is still above 91.94%.

Experimental section

Preparation of CoNiCuMnMo-HEA@rGO

A five-membered high-entropy Co–Cu–Mn–Mo–Ni alloy (CoNiCuMnMo-HEA@rGO) was prepared on rGO. First, 120 mg of graphene oxide was weighed and added to 60 mL mixture solution with an ethanol–water solvent content of V_EtOH/V_H2O = 0.5 and sonicated for 5 hours until a homogeneous graphene oxide dispersion was obtained. Next, CoCl₂·6H₂O, NiCl₂·6H₂O, CuCl₂·2H₂O, MoCl₅, MnCl₂·4H₂O and 2,5-dihydroxyterephthalic acid with a molar ratio of 1 : 1 : 1 : 1 : 1 : 1 : 3 was added to a beaker and mixed with 50 ml of N,N-dimethylformamide (DMF). After complete dissolution, the resulting solution was added to the graphene oxide dispersion and dispersed with ultrasound, and 90 mL of DMF was added. The solution was transferred to a PTFE-lined stainless-steel autoclave during ultrasonication for several hours and then heated at 120 °C for 24 h. The product was subsequently collected and washed several times repeatedly with deionized water and ethanol, and then the resulting product was freeze-dried. A suitable amount of the product was then transferred to a tube furnace (V_Ar/V_H2 = 9 : 1) and heated to 350 °C for 1 hour and continued to 600 °C for 2 hours. The rGO was prepared by the same procedure except that metal salts and ligands were not added.

Preparation of modified separators

Briefly, the modified separators were prepared using a simple coating technique. 80% CoNiCuMnMo-HEA@rGO, 10% Super P, and 10% PVDF were ground in polyvinylidene fluoride solvent (NMP) to get a homogeneous slurry and cast onto the Celgard PP separator, and then dried at 60 °C, recorded as CoNiCuMnMo-HEA@rGO/PP. A slicer with a 19 mm mold was used to cut the prepared separators into small pieces for use. Based on the above method, rGO/PP was prepared as a comparison sample using the same procedure.²⁵

Battery assembly and measurements

In this work, the Li–S battery was assembled with a rGO@S cathode, modified separators (CoNiCuMnMo-HEA@rGO/PP and rGO/PP), a Li anode, and a traditional Li–S electrolyte forming the CR2032 coin-type cell for common electrochemical tests. The coin cell was left to stand for approximately 6 hours after assembly to ensure that the electrolyte completely saturated the electrodes. The charge/discharge and CV testing were conducted on the Neware battery tester (CT-4000) and CHI760E electrochemical workstation, respectively, with a voltage window of 1.7–2.8 V. The EIS test was conducted between 0.01 and 10⁵ Hz in frequency.

Li₂S₆ adsorption experiment

Li₂S and sulfur powders (molar ratio: 5 : 1) were dissolved into the solvent of DME/DOL (v/v, 1 : 1) and stirred at 60 °C for one day to form Li₂S₆ solution (5 mM). 100 mg CoNiCuMnMo-HEA@rGO and rGO were weighed and placed in 10 mL transparent glass, and then the same amount of 0.1 M Li₂S₆ solution was added and left to stand overnight; the color change of different solutions was observed. An equal volume of Li₂S₆ solution was placed in 10 mL transparent glass as a blank control group. The XPS test was conducted to analyze the adsorbed precipitates and UV-vis spectrophotometry tests for analyzing the supernatant.¹³

Li₂S oxidation experiment

100 mg CoNiCuMnMo-HEA@rGO and rGO were respectively placed in 10 mL transparent glass, then 50 mg Li₂S was added, and then 10 mL DME solution was added, and the mixture was continuously stirred for 12 h. After standing for some time, the color change of the upper solution was observed. In addition, 50 mg of Li₂S were placed in transparent glass with 10 mL DME solution and stirred for 10 h as a blank control group. All the above experimental operations were carried out in an argon atmosphere glove box.

Results and discussion

Metal–organic framework compounds were synthesized in situ on graphene oxide nanosheets (GO) by a simple hydrothermal method, using five metal salts of Co, Cu, Mn, Mo and Ni and 2,5-dihydroxyterephthalic acid as raw materials, and then the composite of reduced graphene oxide (rGO) nanosheets anchored by CoNiCuMnMo-HEA (CoNiCuMnMo-HEA@rGO) was obtained by pyrolysis in an Ar/H₂ mixture at high temperature (Fig. 1a). Based on CoNiCuMnMo-HEA@rGO, a high-entropy alloy nano-catalyst with intrinsic thermodynamic stability and multi-component advantages can realize multi-active site design and catalytic performance adjustment in a larger component space.²⁶ The two-dimensional rGO is due to its good electrical conductivity, flexible structure and rich functional groups, thus effectively inhibiting particle aggregation and rapid mass and charge transfer. The above structural analysis implies that CoNiCuMnMo-HEA@rGO will be promising as a catalytic intercalation layer for lithium–sulfur batteries to achieve bi-directional catalytic conversion of polysulfides/sulfides.²⁷ The X-ray diffraction (XRD) pattern exhibits three peaks at 45.1, 51 and 75.7, indicating that the CoNiCuMnMo-HEA nanoparticles have a face-centered cubic (FCC) crystal structure (Fig. S1†).²⁶ The calculated results of the Brunauer–Emmett–Teller (BET) test show that the specific surface area of CoNiCuMnMo-HEA@rGO is 523.4 m² g⁻¹ and rGO does not reduce its own surface area after loading with HEA particles and this continues to maintain the large surface area (Fig. S2†). As shown in Fig. 1b, the scanning electron microscopy (SEM) image clearly shows that the smooth nanosheets of rGO are folded together. From Fig. 1c, it can be seen that a large number of CoNiCuMnMo-HEA nanoparticles are anchored on rGO (Fig. S3†). Moreover, the content of CoNiCuMnMo-HEA in the CoNiCuMnMo-HEA@rGO sample is 51.04 wt% based on the thermogravimetric analysis (TGA) in air (Fig. S4†), demonstrating that there is a high CoNiCuMnMo-HEA loading in the CoNiCuMnMo-HEA@rGO catalyst. The transmission electron microscopy (TEM) image also shows that CoNiCuMnMo-HEA nanoparticles are dispersed on rGO nanosheets (Fig. 1d), and their particle sizes are mainly distributed around 12 nm (inset in Fig. 1d). As shown in Fig. 1e, a lattice spacing of 0.34 nm is observed at the junction of the HEA particles with rGO, corresponding to the (002) lattice plane of carbon,²⁸ indicating that HEA is anchored on rGO. The high-resolution TEM (HRTEM) image shows a clear lattice stripe of HEA with a lattice spacing of 2.04 Å (Fig. 1f), corresponding to the (111) plane of the prepared catalyst.²⁹ Obviously, (111) is a face-centered cubic structure, indicating a stable structure (Fig. 1g). In addition, elemental analysis by energy dispersive spectroscopy (EDS) shows that Co, Ni, Cu, Mn and Mo coexist and are uniformly distributed in the rGO skeleton (Fig. 1h), which further confirms the formation of a single-phase alloy.

Fig. 1 Morphological and structural characterization. (a) Synthesis schematic diagram of a CoNiCuMnMo high entropy alloy anchored on reduced graphene oxide sheets (CoNiCuMnMo-HEA@rGO). (b) rGO and (c) CoNiCuMnMo-HEA@rGO SEM images. (d) TEM images of CoNiCuMnMo-HEA@rGO, the inset is a diameter distribution of HEA nanoparticles. (e and f) HRTEM image of CoNiCuMnMo-HEA@rGO. (g) Schematic diagram of a face-centered cubic structure and close-packed face family. (h) The elemental EDS-mappings, showing uniform mixing of five metal components. (i) Elemental content analysis by using ICP. (j) XPS spectrum of Mn 2p.

Fig. 1i shows that the concentration of each metal in the CoNiCuMnMo-HEA@rGO was analyzed by inductively coupled plasma mass spectrometry (ICP), where the mass ratios of Co, Cu, Mn, Mo, and Ni were 33 : 24 : 13 : 10 : 41. X-ray photoelectron spectroscopy (XPS) was utilized to analyze the surface elemental composition of CoNiCuMnMo-HEA@rGO. The XPS survey spectrum verifies the presence of the elements Co, Cu, Mn, Mo, and Ni (Fig. S5a†). In the high-resolution Mn 2p spectrum of CoNiCuMnMo-HEA@rGO (Fig. 1j), the peaks at 637.54, 641.65, 645.64 and 653.67 eV correspond to Mn⁰ 2p_3/2, Mn²⁺ 2p_3/2, Mn³⁺ 2p_3/2 and Mn²⁺ 2p_1/2,²⁶ respectively. The XPS spectra of high-resolution Cu 2p, Co 2p and Ni 2p show the existence of a metal phase and oxide phase (see Fig. S5† for detailed analysis). Compared with other elements, the highest content of Mn element shows a strong oxidation peak, which indicates that Mn metal exhibits strong activity in CoNiCuMnMo-HEA@rGO.²⁶

In the context of electrochemical reactions in Li–S batteries, the rate of the sulfur reduction reaction (SRR) dictates the duration of the existence of soluble LiPSs, which plays a critical role in the overall performance of the battery.¹¹ In evaluating the catalytic activity of the as-prepared materials, lithium metal was used as the anode, reduced graphene oxide with 70% sulfur content (rGO@S) was used as the cathode (Fig. S6†), and CoNiCuMnMo-HEA@rGO and rGO were used as catalytic inserts coated on the separator (named CoNiCuMnMo-HEA@rGO/PP and rGO/PP) to assemble a button cell, respectively. The effect of CoNiCuMnMo-HEA@rGO on SRR kinetics was studied by comparing with rGO. Notably, CoNiCuMnMo-HEA@rGO/PP and rGO/PP did not reveal any material separation after several manual folding tests, indicating that the macroscopic flexibility of the pristine separator was not altered by loading the coating and that it exhibited good adhesion and mechanical strength (Fig. S7†).

Fig. 2a shows the cyclic voltammetry (CV) curves tested at a scan rate of 0.1 mV s⁻¹ within the voltage range of 1.7–2.7 V. The CV curves all exhibit typical sulfur redox peaks, with two cathodic peaks appearing at approximately 2.3 and 2.0 V, indicating two reduction processes. These can be attributed to sulfur first being reduced to soluble long-chain LiPSs, and then further reduced to insoluble Li₂S/Li₂S₂.³⁰ An anodic peak appears at approximately 2.4 V, which can be attributed to the reversible oxidation of Li₂S₂/Li₂S to LiPSs, and eventually to sulfur.¹² Compared to the rGO/PP (peak voltage difference of 0.40 V), the redox peak-to-peak voltage difference of the CoNiCuMnMo-HEA@rGO/PP cell (0.32 V) is significantly reduced, indicating that the electrochemical polarization of the battery is smaller with the assistance of CoNiCuMnMo-HEA.³¹ In addition, the reduction peak of the CoNiCuMnMo-HEA@rGO/PP CV curve shows a significant shift towards higher voltages, suggesting that soluble LiPSs are rapidly transformed.¹⁶ To gain a more detailed understanding of the catalytic effect of CoNiCuMnMo-HEA@rGO on redox kinetics, the Tafel slope was calculated from the CV curves to evaluate the catalytic effect of CoNiCuMnMo-HEA@rGO.³² As shown in Fig. 2b, the slope for the reduction of S₈ to LiPSs for CoNiCuMnMo-HEA@rGO/PP is 58.9 mV dec⁻¹, which is significantly smaller than that of the rGO/PP cell (81.7 mV dec⁻¹). Additionally, the Tafel slope for the further reduction of LiPSs to Li₂S also reveals that the slope for CoNiCuMnMo-HEA@rGO/PP is the smallest (Fig. 2c). This indicates that HEA can promote the solid–liquid and liquid–solid conversion processes of LiPSs. Meanwhile, the slope for the oxidation of Li₂S to LiPSs for CoNiCuMnMo-HEA@rGO/PP is 57.7 mV dec⁻¹, which is smaller than that for rGO/PP (Fig. 2d). Analysis of the above shows that the Tafel slope for CoNiCuMnMo-HEA@rGO/PP is significantly smaller than that for rGO/PP, both during lithiation and delithiation processes, suggesting that the presence of HEA accelerates the redox reactions of LiPSs, promotes SRR behavior, and accelerates the oxidation of Li₂S.⁸Fig. 2e presents the electrochemical impedance spectroscopy curves tested under open-circuit voltage for CoNiCuMnMo-HEA@rGO/PP and rGO/PP. The CoNiCuMnMo-HEA@rGO/PP cell exhibits a smaller resistance, which is primarily attributed to the introduction of CoNiCuMnMo-HEA facilitating the conversion of LiPSs and further accelerating the reaction kinetics of Li–S batteries. Additionally, symmetric cells were assembled to test the CV curves for further investigation into the electrocatalytic activity and reversibility of CoNiCuMnMo-HEA@rGO towards LiPS conversion.³³ In symmetric cells with 0.1 M Li₂S₆ as the electrolyte, different catalytic electrodes exhibit distinct peak currents in the CV curves. The symmetric cell with CoNiCuMnMo-HEA@rGO as the electrode shows two pairs of oxidation–reduction peaks with larger current and sharper peak shapes, which are attributed to the two-step reduction of Li₂S₆ on the working electrode during the cathodic scan and the oxidation of Li₂S on the counter electrode during the anodic scan. Moreover, the peak currents of the redox reactions are significantly higher than those on the rGO electrode, indicating that the CoNiCuMnMo-HEA@rGO surface exhibits better electrocatalytic activity for the redox reactions of LiPSs (Fig. 2f.³⁴

Fig. 2 Study on catalytic activity. (a) CV curves of various separators at 0.01 mV s⁻¹, and (b–d) Tafel plots. (e) Impedance spectroscopy curves. (f) Li₂S₆ symmetric cell at 1 mV s⁻¹. (g) I_P/v^0.5 value. Precipitation profiles of Li₂S with (h) CoNiCuMnMo-HEA@rGO and (i) rGO.

To compare the battery response current, indicative of the internal lithium-ion diffusion rate, after the catalyst improvement, we assembled symmetric cells and conducted electrochemical testing. The CV curves of CoNiCuMnMo-HEA@rGO/PP and rGO/PP range from 0.1 mV s⁻¹ to 0.6 mV s⁻¹ (Fig. S8†). Notably, plotting the anodic and cathodic peak currents of these cells at varying scan rates against the square root of the scan rate revealed a steeper slope in the Randles–Sevcik plots for cells incorporating CoNiCuMnMo-HEA.³⁵ Evidently, the absolute slope of CoNiCuMnMo-HEA@rGO/PP was significantly higher than that of rGO/PP (Fig. 2g), strongly indicating that HEA nanoparticles significantly enhance the diffusion behavior of Li⁺.

The kinetics of the liquid-to-solid transformation of LiPSs/Li₂S and the ion transfer capabilities were further supported by the Li₂S nucleation and dissociation experiments.³⁶ The nucleation and dissociation processes in the LiPS redox reactions were monitored using chronopotentiometry. Fig. 2h and i depict the constant potential current distribution on the surfaces of CoNiCuMnMo-HEA@rGO and rGO precipitated with Li₂S, respectively. Prior to nucleation, CR2025 coin cells were assembled using CoNiCuMnMo-HEA@rGO and rGO as cathodes and 20 μL Li₂S₈ as the electrolyte. In detail, the nucleation time of Li₂S on CoNiCuMnMo-HEA@rGO (1074 s) was earlier than on rGO (1821 s), which may be attributed to the synergistic interface of CoNiCuMnMo-HEA and rGO with high absorption capacity for LiPSs and rapid reaction kinetics, facilitating the nucleation of Li₂S. Moreover, the precipitation capacity of Li₂S was calculated based on Faraday's law. The precipitation capacity of the CoNiCuMnMo-HEA@rGO electrode (196.5 mA h g⁻¹) is significantly higher than that of the rGO electrode (147.6 mA h g⁻¹), indicating that CoNiCuMnMo-HEA@rGO can significantly promote the liquid-to-liquid and liquid-to-solid transformations of LiPSs compared to the rGO matrix.

Furthermore, to visually demonstrate the interaction between CoNiCuMnMo-HEA@rGO and LiPSs, we conducted an adsorption experiment with Li₂S₆. As evidenced by the optical images, over time, the initially pale yellow Li₂S₆ solution became transparent in the presence of CoNiCuMnMo-HEA@rGO, whereas the pale yellow color remained clearly observable in the rGO (see the inset of Fig. S9a†). This phenomenon indicates that CoNiCuMnMo-HEA@rGO possesses a stronger physical adsorption capacity for LiPSs compared to the original rGO. Additionally, ultraviolet-visible (UV-vis) spectroscopy analysis of the adsorbed solution (Fig. S9a†) shows a significant decrease in the intensity of the characteristic peak at approximately 260 nm, corresponding to S₆²⁻, indicating its strong physical adsorption capability for LiPSs, consistent with the aforementioned analyses. Subsequently, the strong interaction between Li₂S₆ and CoNiCuMnMo-HEA@rGO was further verified by XPS (Fig. S9b–f†). It was observed that after adsorbing Li₂S₆, the peaks of Co 2p and Ni 2p shifted towards higher binding energies, indicating an increase in the electron density at the Co and Ni centers. This is due to the chemical interaction of Co and Ni with Li₂S₆.^15,37 In the XPS spectrum of Mo, the characteristic peak of Mo⁶⁺ decreased after the adsorption experiment, indicating a strong chemical reaction between Mo⁶⁺ and Li₂S₆. Additionally, the position of the Mo 3d peak shifted slightly towards higher binding energy, suggesting a strong electronic interaction between Mo and LiPSs.³⁸ The binding energies corresponding to Mn, Cu, and their respective ions (Mn²⁺ and Cu²⁺) were significantly higher than those before adsorption, indicating interactions between Mn, Cu, and LiPSs. These results further confirm that the polar adsorption between CoNiCuMnMo-HEA@rGO and LiPSs leads to intense chemical anchoring and effectively captures LiPSs. In addition, Fig. S10† shows the ex situ SEM and maps of S elements on the electrode containing different modified separators with the distribution. For the electrode containing CoNiCuMnMo-HEA@rGO/PP, the S element is uniformly distributed on the rGO surface. In Fig. S10b,† for the electrode containing rGO/PP, the sulfur content almost disappears in the middle of the discharge. The results show that CoNiCuMnMo-HEA@rGO has a good adsorption capacity for polysulfide and can well prevent LiPSs from passing through the separator.

The catalytic performance of the CoNiCuMnMo-HEA@rGO catalyst for the reverse reaction in Li–S batteries, specifically its oxidation catalytic effect on Li₂S, was investigated using UV-vis and XPS techniques to analyze the interaction between Li₂S and CoNiCuMnMo-HEA@rGO. Compared with Li₂S and rGO (inset of Fig. 3a), the supernatant of the reaction between the insoluble Li₂S and CoNiCuMnMo-HEA@rGO exhibited a significant color change (from colorless to yellowish-green), indicating the formation of LiPSs³⁹ (the black object at the bottom is the magnetic stirrer). From Fig. 3a, the presence of LiPSs was further confirmed by UV-vis spectroscopy, which showed characteristic peaks at approximately 260 nm for S₆²⁻ and at 375 nm for S₄²⁻.

Fig. 3 Study of Li₂S oxidation processes. (a) Uv–vis spectra and optical photographs of different samples in contact with Li₂S. XPS spectra of (b) Co 2p, (c) Cu 2p, (d) Mn 2p, (e) Mo 3d, and (f) Ni 2p before and after the interaction of CoNiCuMnMo-HEA@rGO with Li₂S. The Li₂S dissociation curve of (g) CoNiCuMnMo-HEA@rGO and (h) rGO. (i) Schematic illustration of the accelerated oxidation process of Li₂S.

It was observed that after the reaction between CoNiCuMnMo-HEA@rGO and Li₂S, the characteristic peaks of metallic Co, Ni, Mo, Mn, and Cu in the XPS spectra shifted. Specifically, the Co²⁺ 2p_1/2 peak shifted to a higher binding energy by 0.4 eV, while the Co⁰ 2p_3/2 and Co⁰ 2p_1/2 peaks shifted to a lower binding energy by 0.7 and 0.9 eV (Fig. 3b), respectively. For Cu, the metallic oxidation states Cu²⁺ 2p_3/2 and Cu²⁺ 2p_1/2 shifted to higher binding energies by 0.3 and 0.35 eV (Fig. 3c), respectively. From Fig. 3d, Mn exhibited three valence states, and all six characteristic peaks shifted to various degrees. Among them, Mn⁰ 2p_3/2 and Mn⁰ 2p_1/2 shifted to higher binding energies by 0.35 and 0.85 eV, while Mn²⁺ 2p_3/2, Mn²⁺ 2p_1/2, Mn³⁺ 2p_3/2, and Mn³⁺ 2p_1/2 shifted by 0.3, 0.25, 0.6, and 1.2 eV, respectively. In Fig. 3e, the Mo⁶⁺ 3d_5/2 and Mo⁶⁺ 3d_3/2 peaks also shifted to higher binding energies by 0.4 and 0.5 eV, respectively. The oxidation states Ni 2p_3/2 and Ni 2p_1/2 orbitals shifted by 0.35 and 0.25 eV (Fig. 3f), respectively. The areas of the oxidized metal states all increased, indicating a reduction in the content of these states after reacting with polysulfides. This demonstrates that CoNiCuMnMo-HEA@rGO form a strong interaction with polysulfides during the oxidation of Li₂S, effectively promoting the formation of polysulfides and accelerating the redox reaction of Li₂S, thereby enhancing the performance of Li–S batteries.¹⁵

To investigate the superiority of CoNiCuMnMo-HEA@rGO in the Li₂S dissociation process, a similar battery configuration was used for constant current discharge to 1.7 V (to ensure that LiPSs were almost completely converted to Li₂S), followed by constant potential charging at 2.35 V. The current response of the CoNiCuMnMo-HEA@rGO electrode (Fig. 3g) was higher than that of the rGO electrode (Fig. 3h), indicating that CoNiCuMnMo-HEA@rGO can effectively reduce the overpotential during the reverse transformation of Li₂S, facilitating the decomposition of Li₂S. Additionally, compared to rGO (265.6 mA h g⁻¹), the CoNiCuMnMo-HEA@rGO electrode (300.4 mA h g⁻¹) exhibited a higher dissociation capacity, demonstrating that CoNiCuMnMo-HEA@rGO promotes the dissociation of Li₂S. These results confirmed that the existence of CoNiCuMnMo-HEA enhanced the reaction kinetics of the transformation of Li₂S into polysulfide/sulfur (Fig. 3i).

Based on our experimental characterization, we utilized a simplified CoNiCuMnMo-HEA particle model to perform density functional theory (DFT) computational simulations, further elucidating the unique advantages of CoNiCuMnMo-HEA in catalyzing the SRR at the atomic level. To this end, we simulated the adsorption behavior of cyclic S₈ and Li₂S_n (1 ≤ n ≤ 8) on CoNiCuMnMo-HEA@rGO and rGO (Fig. S11†), respectively. Compared to rGO, the Li₂S_n (1 ≤ n ≤ 8) adsorbed by CoNiCuMnMo-HEA@rGO exhibited slight or obvious deformation, primarily due to the formation of chemical bonds between the S atoms and the metal atoms. This is also important proof that CoNiCuMnMo-HEA can effectively adsorb LiPSs and suppress shuttle effects. For the multi-phase catalytic conversion of LiPSs, the tight adsorption of active species on catalytically active sites is a prerequisite.⁴⁰ As can be seen from Fig. 4a, the binding energies of various LiPSs on the surface of CoNiCuMnMo-HEA@rGO based on DFT calculations are much stronger than those on rGO, and for soluble Li₂S_n, CoNiCuMnMo-HEA@rGO exhibits a much stronger binding energy, which is consistent with the results of the adsorption experiments.

Fig. 4 Density functional theory calculation. (a) Binding energy of various S species adsorbed on HEA@rGO and rGO. (b) Gibbs free energy of the sulfur reduction process on HEA@rGO and rGO. (c) Density of states of various metals after interaction with Li₂S₆.

In addition, we further considered the transformation pathways of intermediates such as *S₈, *Li₂S₈, *Li₂S₆, *Li₂S₄, *Li₂S₂, and *Li₂S to evaluate the catalytic performance of CoNiCuMnMo-HEA@rGO (Fig. 4b). Compared to rGO, the presence of CoNiCuMnMo-HEA@rGO led to a significant reduction in the overall free energy of LiPSs and greatly decreased the energy barriers for each step of their transformation, indicating that CoNiCuMnMo-HEA@rGO accelerates the catalytic conversion of LiPSs. This demonstrates that the multi-metal synergistic effect of CoNiCuMnMo-HEA is the core to accelerate the catalytic conversion of LiPSs.⁴¹ Additionally, we calculated the density of states (DOS) of Li₂S₆ on different substrates and found that compared to other single metal substrates, CoNiCuMnMo-HEA has more dense energy bands, which are more conducive to electron motion and transition, indicating a stronger interaction between Li₂S₆ and CoNiCuMnMo-HEA. This can be attributed to the renowned cocktail effect of CoNiCuMnMo-HEA (Fig. 4c). These analyses indicate that CoNiCuMnMo-HEA, due to the multi-metal synergistic catalysis resulting from the cocktail effect, enhances the adsorption of LiPSs, accelerates redox reactions, and thereby effectively suppresses shuttle effects.

Fig. 5a presents the CV curve of the CoNiCuMnMo-HEA@rGO/PP battery obtained at a scanning rate of 0.1 mV s⁻¹, from which it can be clearly observed that the cathodic and anodic peak curves in the first three cycles are almost identical, indicating that the catalytic effect of CoNiCuMnMo-HEA@rGO/PP exhibits good chemical reversibility and stability. Fig. 5b shows the charge–discharge curve of the battery at a rate of 0.2 C. The discharge capacity at the second discharge plateau of the CoNiCuMnMo-HEA@rGO/PP battery significantly increased, indicating that the conversion efficiency from LiPSs (Li₂S_n, 4 ≤ n ≤ 8) to Li₂S/Li₂S₂ has been significantly improved. Notably, the potential difference (ΔE₁) between the second charge and discharge curves of the CoNiCuMnMo-HEA@rGO/PP is relatively smaller than that of the rGO/PP, indicating that the battery with a CoNiCuMnMo-HEA functional separator has lower polarization and better reversibility. Subsequently, at different current rates from 0.2 to 5 C, the HEA@rGO/PP battery exhibited a higher discharge specific capacity (Fig. 5c). The initial discharge specific capacity of HEA@rGO/PP was 1512.4 mA h g⁻¹. When the current density increased to 5 C, the discharge capacity stabilized at 570.2 mA h g⁻¹. When the current density was restored to 0.2 C, the battery capacity rebounded to 796.5 mA h g⁻¹, demonstrating good chemical reversibility and a high utilization rate of the active material (Table S1†). The constant current charge–discharge voltage curves of CoNiCuMnMo-HEA@rGO/PP and rGO/PP batteries between 1.5 and 3.0 V showed two discharge plateau regions (Fig. S12†), corresponding to the conversion of cyclic S₈ to LiPSs, which then transform into solid Li₂S₂/Li₂S. In addition, the thickness of catalytic intercalation has a significant influence on the performance (Fig. S13†), and the best performance is obtained when the intercalation thickness is about 20 μm.

Fig. 5 Electrochemical performance analysis of separator-assembled batteries. (a) CV curves of CoNiCuMnMo-HEA@rGO/PP during three cycles. (b) Charge–discharge profiles at 0.2 C. (c) Rate capabilities of the different rates for CoNiCuMnMo-HEA@rGO/PP and under S loading of 1.0 mg cm⁻². (d) Comparison of the recently reported electrochemical performance of the high entropy material for the Li–S battery. (e) Cycle performance and the corresponding coulombic efficiency. (f) Electrochemical performance of CoNiCuMnMo-HEA@rGO/PP under high sulfur loading.

As shown in Fig. 5d, CoNiCuMnMo-HEA@rGO/PP shows excellent rate performance compared with HEA-based lithium–sulfur batteries reported in the literature.^24,42–50Fig. 5e shows the long-term cycle performance tested at 3 C with different interlayers when the sulfur loading is 1.0 mg cm⁻². CoNiCuMnMo-HEA@rGO/PP exhibits experienced 1100 cycles, and the decay rate of each cycle is only 0.055%, and the Coulomb efficiency exceeds 91.94%, which is significantly better than that of rGO/PP. As shown in Fig. S14,† compared to the pristine lithium metal with a metallic luster, the surface topography of the lithium metal electrodes becomes rough as the current density increases. Moreover, after cycling for 100 cycles at 3C, many black spots appear on the surface of the lithium metal electrodes, indicating the formation of ‘dead lithium’, which has been believed to be one of the most important reasons for the decline of coulombic efficiency. The above excellent long-cycle performance is attributed to the high catalytic activity and structural stability of the close-packed catalyst surface with a face-centered cubic structure. Tests were conducted on batteries with high sulfur loadings (Fig. 5f). The initial area specific capacity of the HEA@rGO/PP battery at 0.1 C reached 2.63 mA h cm⁻², and the capacity remained at 2 mA h cm⁻² after 100 cycles. This is mainly due to the fact that CoNiCuMnMo-HEA@rGO/PP can accelerate the bidirectional reaction power of polysulfide/sulfide, thus effectively inhibiting the shuttle effect and improving the battery performance.

Based on the bidirectional catalytic conversion of polysulfides/sulfides by CoNiCuMnMo-HEA, it can effectively inhibit the shuttle effect of polysulfides and reduce the generation of lithium dendrites and dead lithium.⁵¹ The formation of Li₂S on the anode side is a result of the slow shuttling of LiPSs to the lithium side, which reacts with lithium to form ‘dead sulfur’ on the one hand, leading to irreversible capacity degradation, and generating ‘dead lithium’ and electrolyte loss on the other hand, thus resulting in the polarization increasing and performance degradation of Li–S batteries. The batteries after cycling were subjected to EIS tests to monitor the trend of external impedance. As shown in Fig. S15,† according to the equivalent circuit model, R_ct is the charge transfer resistance resulting from the reaction kinetics of the conversion of long-chain LiPSs to short-chain Li₂S₂/Li₂S on the cathode corresponding to the semicircle in the mid- to high-frequency region. R_SEI is the solid electrolyte interface (SEI) resistance. The post-cycling EIS shows two semicircles in the high and mid-frequency regions. The formation of R_SEI indicates the resistance of Li⁺ in the SEI film, confirming the generation of a stable SEI at the electrolyte interface in contact with lithium metal. The presence of the SEI film prevents the continuous electron transfer between the Li anode and the LiPSs in the electrolyte, thus stabilizing the surface structure of the Li-anode. The total impedance of the Li–S battery decreased after cycling, which could be attributed to the redistribution of sulfur in the cathode. The EIS results confirmed that CoNiCuMnMo-HEA@rGO could induce the uniform generation of the SEI layer, which reduces the ‘dead lithium’, improves the utilization rate of the active lithium, and ultimately prolongs the cycle life of the battery. To verify this hypothesis, the battery was disassembled after 100 cycles, and the corrosion degree of the lithium anode was observed using scanning electron microscopy. The anode corresponding to rGO/PP suffered severe corrosion compared to CoNiCuMnMo-HEA@rGO/PP (Fig. S12†). These analyses indicate that introducing CoNiCuMnMo-HEA into the intercalated separator can effectively accelerate the catalytic conversion of LiPSs, inhibit the formation of lithium dendrites, and significantly improve the performance of Li–S batteries.

Conclusion

In summary, we synthesized CoNiCuMnMo-HEA nanoparticles by anchoring five metal–organic framework compounds, Co, Ni, Cu, Mn, and Mo, to rGO using hydrothermal and high-temperature cracking methods, which showed excellent catalytic activity as catalytic intercalation layers for Li–S batteries. LiPS reaction kinetics are significantly improved and active material utilization is improved, when strongly coupled CoNiCuMnMo-HEA caused charge redistribution to form a good electronic structure. According to DFT calculations, LiPSs can be effectively immobilized and the shuttle effect can be suppressed by the multimetallic composite CoNiCuMnMo-HEA by lowering the Fermi energy level between sulfur and the alloy particles. Consequently, the CoNiCuMnMo-HEA@rGO/PP exhibits a remarkable specific capacity decay rate of only 0.055% per cycle after 1100 cycles at a current density of 3 C, with a coulombic efficiency consistently maintained above 91.94%. This work expands the design philosophy and strategy for multifunctional catalysts applied in high-performance Li–S batteries.

Author contributions

D.G., X.C. and S.W. conceived and designed the research; X.W., P.L. and D.G. performed the experiments and the characterization of the materials; C.W. (Chuanhuang Wu) conducted part of the characterization; Y.Z. and C.W. (Cong Wang) performed part of the synthesis and electrochemical tests; X.W. and P.L. co-completed the writing of the original draft; D.G. made instructive advice to revise the full text and supported the perfection of the manuscript; D.G., X.C. and S.W. are responsible for funding acquisition. All authors contributed to the discussion of the manuscript.

Data availability

Data openly available in a public repository.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was financially sponsored by the National Natural Science Foundation of China (52271225 and 52331009), the Natural Science Foundation of Zhejiang Province (LQ23E020001), the Baima Lake Laboratory Joint Funds of the Zhejiang Provincial Natural Science Foundation of China (LBMHZ24B060006), the Basic Scientific Research Projects of Wenzhou City (G20220024, H20220002), and the Doctoral Innovation Foundation of Wenzhou University (3162024001005).

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Footnotes

† Electronic supplementary information (ESI) available: Detailed characteristics of the catalysts and additional reduction results. See DOI: https://doi.org/10.1039/d4qi01434k

‡ These authors contributed equally to this work.

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