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High entropy sulfide nanoparticles as redox catalysts for lithium–sulfur batteries†

Yiqian Li‡ ^a, Yuehan Hao‡^a, Usman Ali^a, Mingxin Sui^c, Qi Zhang*^b, Lingyu Zhang^a, Lu Li^a, Bingqiu Liu*^a and Chungang Wang^a
aFaculty of Chemistry, Northeast Normal University, Changchun, 130024, P. R. China. E-mail: liubq142@nenu.edu.cn
^bDepartment of Resources and Environment, Jilin Agricultural University, Changchun, 130118, P. R. China. E-mail: zhangq055@nenu.edu.cn
^cState Grid Corporation of China (SGCC), State Grid Liaoning Electric Power Co., Ltd. Dandong Power Supply Company Donggang Branch, Donggang, 118300, P. R. China

First published on 30th April 2025

1. Introduction

In response to the aforementioned challenges, researchers have effectively mitigated the shuttle effect of LiPSs by introducing catalysts for chemisorption and anchoring of soluble polysulfides.^20–23 However, the scarcity of active sites restricts the catalytic performance of these simple component catalysts, rendering them insufficient for facilitating the intricate redox process involving 16 electron conversions.²⁴ In addition, the poor electrochemical stability of certain catalysts significantly compromises the cycling performance of Li–S batteries.^25–29 Therefore, it is of paramount importance to acquire a catalyst exhibiting exceptional catalytic activity, remarkable stability, and broad applicability for boosting the performance of Li–S batteries.

High entropy sulfide (HES) typically consists of five or more elements randomly distributed throughout the crystal lattice.^30–33 The presence of uniformly mixed and dispersed various metal cations will produce a ‘cocktail effect’, resulting in diverse chemical and electronic adsorption sites, which can catalyze the intricate chemical reactions of Li–S batteries and effectively inhibit the shuttle effect.^34,35 For instance, Theibault et al. reported a Zn_0.30Co_0.31Cu_0.19In_0.13Ga_0.06S nanoparticle catalyst, which effectively enhances the rate of LiPSs redox reaction and mitigates the shuttle effect of LiPSs.³⁶ Generally, the catalytic performance is closely related to the morphology and microstructure of materials, which are highly dependent on the synthesis method employed. However, the conventional approach for synthesizing HES typically requires elevated temperatures to guarantee the homogeneous blending of diverse elements possessing dissimilar atomic radii, valence states, and oxidation potentials, which directly impacts the morphology of HES. The ultrafine HES nanocrystals, in contrast, not only expose a sufficient number of active sites but also ensure effective polysulfide capture and rapid redox reaction kinetics. Therefore, it is of great significance to prepare ultrafine HES as a separator modifier to make a breakthrough in the application of Li–S batteries.

Herein, we prepared ultrafine FeCoNiCrMnS₂ nanospheres (HES NSs) using a facile template method and utilized them as a separator modifier in Li–S batteries. The ultrafine HES nanocrystals can expose a sufficient number of active sites, which can serve as an efficient LiPSs barrier to inhibit side reactions and as an additional collector to enhance the polysulfide redox reaction. Therefore, the battery employing the HES//PP separator exhibits outstanding electrochemical performance. At a current density of 1.0 C, the initial discharge capacity reaches 1098.3 mA h g⁻¹. After 1000 cycles, the high discharge specific capacity of 906.8 mA h g⁻¹ is still maintained, with a negligible capacity decay rate per cycle of only 0.017%. This study not only demonstrates the application potential of HES as a material for modifying separators in Li–S batteries but also presents novel insights for investigating other high-entropy materials.

2. Experimental

2.1 Materials

2.2 Synthesis

2.2.2 Synthesis of HES//PP modified separator. The as-synthesized HES NSs, carbon nanotubes (CNTs), and poly-vinylidene fluoride (PVDF) with a mass ratio of 8:1:1 were dispersed in N-methyl-2-pyrrolidinone (NMP) by stirring for 8 h, and then the slurry was coated onto polypropylene (PP) separator (Celgard 2500) to obtain the modified separators, denoted as HES//PP. After drying, the separators were cut into circular disks with a diameter of 16 mm.

3. Results and discussion

	Fig. 1 (a) The HES NS synthesis diagram. TEM images of (b) PAA-NH4 NSs, (c) [Fe/Cr](OH)3/[Co/Ni/Mn](OH)2/PAA-NH4 NSs, (d and e) HES NSs. (f) HR-TEM image of HES NS. (g and h) SEM images of HES NSs. (i) SAED patterns of HES NSs. (j–o) Elemental distribution of HES NSs.

The TEM and SEM were employed to analyze the morphology and microstructure of the materials under investigation. Fig. 1b displays the TEM images of PAA-NH₄ NSs, while Fig. 1c shows the TEM images of [Fe/Cr](OH)₃/[Co/Ni/Mn](OH)₂/PAA-NH₄ NSs with an approximate particle size of 200 nm. The HES NSs obtained by the solvothermal method maintain a shape and size similar to the precursor, with nanocrystals measuring approximately 10 nm. (Fig. 1d and e). The HR-TEM images in Fig. 1f exhibit clear lattice fringes, exhibiting a measured lattice spacing of 0.271 nm, which corresponds to the crystal plane (200) of pyrite FeS₂. The spherical morphology and intact structure of HES NSs can be observed in the SEM images presented in Fig. 1g and h. The selected area electron diffraction (SAED) image depicted in Fig. 1i exhibits distinct diffraction rings corresponding to the crystal planes of (111), (200), (211), (220), (222), and (311). This compelling evidence further substantiates the formation of a single-phase spinel structure. The elemental distribution map of HES NSs is presented in Fig. 1j–o, revealing a uniform distribution of various elements within the HES NSs. Fig. S1† is the morphology characterization image of FeS₂ NSs.

The XRD pattern in Fig. 2a shows that all diffraction peaks of HES NSs correspond well to the FeS₂ standard card (JCPDS No. 71-0053), which proves that we have successfully prepared multi-component single-phase structure of high-entropy sulfides.³⁷ Additionally, Fig. S2† depicts the XRD of FeS₂ NSs, indicating the successful synthesis of FeS₂ NSs. The N₂ adsorption/desorption isotherms for HES NSs are presented in Fig. S3.† The Brunauer–Emmett–Teller (BET) specific surface area of the HES NSs is measured to be 54.107 m² g⁻¹. HES NSs were characterized and analyzed using ICP-AES, and the atomic ratio of HES NSs was determined to be 1:1:1:1:1 (Table S1†). In order to further investigate the performance of HES NSs as a modifier for separators, we coated it on a commercial polypropylene (PP) separator to prepare an HES//PP separator. The SEM image in Fig. 2b reveals that the modified layer of HES NSs on the PP separator has an approximate thickness of 4.5 μm. The presence of HES NSs towards the sulfur cathode side impedes the migration of polysulfides through the separator, while the PP separator facing the lithium metal anode side acts as a barrier to prevent short circuits. Furthermore, the HES//PP separator exhibits a contact angle of nearly 0° with the electrolyte, whereas the contact angle of the commercial PP separator is 37.4° (Fig. 2c). The test results confirm that the HES//PP separator exhibits favorable electrolyte wettability, thereby facilitating enhanced migration of Li⁺. The adhesion and mechanical properties of the HES//PP separator have been significantly improved through modifications, as depicted in Fig. 2d. Notably, the coating remains intact even after repeated folding and unfolding. The SEM images in Fig. 2e and f depict the CNTs before and after sulfur loading, respectively. A layer of sulfur nanoparticles is observed to be attached to the surface of CNTs, resulting in a roughened surface morphology, thus confirming successful sulfur loading onto the CNTs.

	Fig. 2 (a) XRD pattern of HES. (b) Cross-section SEM image of the HES//PP separator. (c) Contact angles of the pristine PP and HES//PP separators with the electrolyte. (d) HES//PP separator under mechanical stability tests. SEM images of (e) CNTs and (f) CNTs/S.

To investigate the practical performance of the HES//PP separator in batteries, a CR2025 coin cell was assembled with CNTs/S cathode and lithium metal anode. The thermogravimetric analysis (TGA) depicted in Fig. S4† reveals that the sulfur content on the CNTs/S cathode is 79.1 wt%. The cyclic voltammetry (CV) curves at 0.1 mV s⁻¹ display a pair of reduction peaks and two noticeable oxidation peaks within the voltage range of 1.7–2.8 V (Fig. S5a†). The CV curves of the initial three cycles exhibit significant overlap, indicating exceptional cycle stability and highly reversible redox reactions in the HES//PP battery. Fig. 3a illustrates the CV profile of the battery employing various separators while maintaining a scanning rate of 0.1 mV s⁻¹. The peak A corresponds to the reduction of elemental sulfur into long-chain polysulfides, and peak B is the reduction process of long-chain polysulfides to Li₂S₂ and Li₂S. The peak C represents the conversion of Li₂S and Li₂S to long-chain polysulfides, followed by their further transformation into elemental sulfur. In comparison to the FeS₂//PP battery, the HES//PP battery exhibited a noticeable positive shift in reduction peak and a negative shift in oxidation peak, indicating that the HES//PP separator has the potential to improve the kinetics of redox reactions involving polysulfides and mitigate their shuttle effect. Fig. 3b and c display Tafel curves derived from the peaks of CV curves across various voltage windows. The HES//PP battery exhibited a lower Tafel slope, thereby confirming the pronounced catalytic conversion effect of HES NSs on polysulfides. According to Fig. 3d, it can be observed that the HES//PP battery exhibits a comparatively lower potential difference (ΔE) compared to the FeS₂//PP battery, indicating a lower polarization in the HES//PP battery and demonstrating excellent electron transport capacity of the HES//PP separator. The electrochemical performance of the HES//PP was further characterized by electrochemical impedance spectra (EIS). Fig. S6† displays the corresponding Nyquist plots, indicating that HES can significantly enhance the transfer of electrolyte ions and electrons at the electrode-solution interfaces. Fig. S5b† presents the CV curve of the HES//PP battery at various scanning rates.

	Fig. 3 (a) CV curves at 0.1 mV s−1. (b and c) CV curves under different voltage windows, with Tafel plots for peaks A and B as illustrated. (d) The difference in potential at the corresponding peak. (e) The charge/discharge curves of HES//PP and FeS2//PP battery at 0.1 C. (f) Rate capability from 0.1 to 5.0 C, and then back to 0.1 C. (g) The long-term cycling performance of Li–S batteries employing various separators at 1.0 C. (h) The cycle performance of HES//PP battery at 0.2 C and a sulfur loading of 5.0 mg cm−2.

Fig. 3e illustrates the galvanostatic charge/discharge curves of the HES//PP and FeS₂//PP battery at 0.1 C. Q₁ and Q₂ denote the specific capacities of the two primary discharge platforms, respectively. It can be seen from the figure that the Q₁ specific capacity of the HES//PP battery is 407.1 mA h g⁻¹, substantially higher than the FeS₂//PP battery's 223.9 mA h g⁻¹, thereby substantiating the superior sulfur utilization rate and accelerated redox reaction rate exhibited by the HES//PP battery. Furthermore, the polarization of the HES//PP battery exhibits a lesser magnitude when compared to that observed in the FeS₂//PP battery, providing further evidence for the enhanced redox reaction kinetics facilitated by HES NSs. The rate performance of HES//PP and FeS₂//PP batteries is depicted in Fig. 3f. The discharge capacities at 0.1 C to 5.0 C are recorded as follows: 1237.9, 1086.3, 956.6, 856.5, 791.3 and 625 mA h g⁻¹ respectively. Furthermore, restoring the current density to 0.1 C reverts the discharge capacity to 1060.3 mA h g⁻¹, demonstrating exceptional reversibility. The charge and discharge profiles of HES//PP batteries under galvanostatic conditions at various current densities are depicted in Fig. S5c.† Remarkably, even when subjected to 5.0 C, the characteristic charge and discharge plateau remain consistently preserved. Fig. 3g illustrates the comparative long-term performance of batteries employing different separators at 1.0 C. The HES//PP battery's initial discharge capacity is 1098.3 mA h g⁻¹, and after 1000 cycles, it retains a high capacity of 906.8 mA h g⁻¹. This superior performance stems from the multi-metal synergy in HES, which simultaneously optimizes polysulfide adsorption and redox kinetics, whereas single/dual-metal sulfides suffer from limited active site diversity. In consideration of the practical application of Li–S batteries, we conducted electrochemical performance testing on HES//PP batteries with a high sulfur loading of 5.0 mg cm⁻². After 200 cycles at 0.2 C, the specific discharge capacity remains at 814.4 mA h g⁻¹ (Fig. 3h). Fig. S7† is the SEM image of the HES//PP after 200 cycles of 0.2 C discharge/charge cycle. It is evident that the structure of the HES NSs is well preserved, which suggests their exceptional mechanical stability. In addition, we compare the performance of this work with several reported transition metal sulfide composites, and the resulting analysis is presented in Table S2.† In comparison to other transition metal sulfide composites documented in the literature, our HES//PP battery exhibits superior long-cycle performance.

We compared the redox ability of different samples to Li₂S₆ by testing symmetrical batteries. It can be seen from Fig. 4a that the redox current of Li₂S₆ in the HES battery is significantly higher compared to that in the FeS₂ battery, indicating a pronounced catalytic conversion effect of HES NSs on the redox process of Li₂S₆. To further investigate the catalytic effect of HES NSs on polysulfides, a Li₂S deposition test was carried out on HES NSs. Fig. 4b is the constant potential discharge curve of Li₂S deposition on different samples. Compared to FeS₂ NSs, the HES NSs exhibit the shortest Li₂S nucleation time, demonstrating their impressive capacity to decrease the excessive potential required for Li₂S nucleation and expedite the formation of Li₂S. Furthermore, the sample's ability to adsorb polysulfides is of utmost importance in reducing the shuttle effect. In order to more intuitively evaluate the adsorption capacity of HES NSs towards polysulfides, the HES NSs, CNTs, and FeS₂ NSs were added to the Li₂S₆ solution for the static adsorption test. As shown in Fig. 4c, the Li₂S₆ solution of HES NSs changed from yellow to almost colorless after 12 h, indicating robust adsorption capability. In contrast, the Li₂S₆ solution added with FeS₂ NSs and CNTs changed little, consistent with the results obtained from theoretical calculations. The results of the UV-vis absorption spectra presented in Fig. S8† further substantiate this conclusion. To investigate the ability of HES NSs modified layer to suppress the shuttle effect of polysulfides, the H-shaped quartz device was employed to test the inhibition of the PP separator and HES//PP separator on polysulfide diffusion. As shown in Fig. S9,† distinct separators were employed to separate a 5 mM Li₂S₆ solution and a pure DME solution. Over a period of time, Li₂S₆ passes through the PP separator, and the solution in the right glass tube turned yellow after 3 h. In contrast, the H-shaped quartz device with HES//PP separator only has a small amount of Li₂S₆ permeation after 24 h, demonstrating the pronounced inhibitory and obstructive effect of HES NSs on polysulfide diffusion.

	Fig. 4 (a) Polarization curve of symmetrical battery. (b) The constant potential discharge curve of Li2S deposition. (c) The adsorption properties of different samples on Li2S6 solution. XPS spectra of HES NSs before and after LiPSs adsorption, (d) Fe 2p, (e) Co 2p, (f) Ni 2p, (g) Cr 2p, (h) Mn 2p and (i) S 2p.

The interaction between the HES NSs modified layer and polysulfides was investigated using X-ray photoelectron spectroscopy (XPS). In the fine spectrum of Fe 2p, as depicted in Fig. 4d, the peaks observed at 707.5 eV and 721.1 eV are indicative of the presence of Fe²⁺, while the peaks detected at 712.3 eV and 724.5 eV can be associated with Fe³⁺.³⁸ The Co 2p fine spectrum in Fig. 4e can be divided into Co³⁺ (779.2 eV and 794.5 eV) and Co²⁺ (782.5 eV and 799.1 eV), along with the presence of satellite peaks at 787.4 eV and 803.8 eV.³⁹ The distinct peaks observed at 854.2 eV, 856 eV, 871.6 eV, and 876.5 eV in the Ni 2p spectrum can be attributed to the presence of Ni 2p_3/2 and Ni 2p_1/2 orbitals, respectively. Moreover, two additional satellite peaks are evident at 860.7 eV and 880.6 eV (Fig. 4f).⁴⁰ The Cr 2p fine spectrum exhibits four characteristic peaks, with the peaks at 577.5 and 587.1 eV corresponding to Cr³⁺, and the peaks at 580.2 eV and 589.5 eV are indicative of Cr⁶⁺ (Fig. 4g).⁴¹ The fine spectrum of Mn 2p is depicted in Fig. 4h, which further verifies the coexistence of Mn³⁺ (642.2 eV and 653.4 eV) and Mn⁴⁺ (645.2 eV and 657.5 eV).⁴² In Fig. 4i, three characteristic peaks with a binding energy of 162.5 eV, 163.8 eV, and 169.2 eV in the fine spectrum of S 2p are attributed to S 2p_3/2, S 2p_1/2 and SO₄²⁻, respectively.⁴³ After adsorption with polysulfides, the characteristic peaks of all elements exhibited distinct shifts to varying degrees, indicating a strong chemical interaction between HES NSs and polysulfides.

The anchoring ability of HES and FeS₂ to polysulfides was investigated using density functional theory (DFT). Fig. 5a and b show the optimal geometry and adsorption energy of polysulfides on HES (200) and FeS₂ (200) surfaces, respectively. According to the computational results, it can be inferred that HES exhibits strong chemical adsorption and capture ability for polysulfides due to the ‘cocktail effect’ of high-entropy materials. The depiction of the Gibbs free energy for the transformation from S₈ to Li₂S on HES and FeS₂ can be observed in Fig. 5c. The step that determines the rate of the sulfur reduction reaction (SRR) is the transformation from Li₂S₄ to Li₂S in a liquid–solid conversion process, which determines the final Li₂S yield. Although the energy barrier for HES in the reduction process from Li₂S₄ to Li₂S is higher compared to that of FeS₂, the whole reduction process of HES is a spontaneous exothermic reaction. The calculation results demonstrate that HES facilitates the sulfur reduction reaction process and exhibits enhanced catalytic activity towards polysulfides. Fig. 5d depicts the HES//PP separator's polysulfide capture and conversion mechanism. The catalytic activity and structural stability of HES NSs are significantly enhanced due to the synergistic effect and high entropy effect exerted by various metal cations. Moreover, the surface exhibits a diverse array of chemical adsorption sites that possess robust capabilities for both chemical adsorption and efficient capture of polysulfides. Based on the above advantages, the battery using the HES//PP separator exhibits exceptional rate performance and cycle stability.

	Fig. 5 (a) Geometry of polysulfides on HES (200) and FeS2 (200) surfaces. (b) Adsorption energy of polysulfides. (c) The Gibbs free energy from S8 to Li2S. (d) The schematic diagram illustrates the capture and conversion mechanism of LiPSs by HES//PP separator.

4. Conclusions

Author contributions

Data availability

Conflicts of interest

Acknowledgements

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