High mixing entropy of MnFeCoNiCu–S to drive high performance sodium storage

Abstract

Transition metal sulfides (TMSs) are often used as anode materials in sodium-ion batteries (SIBs). Nevertheless, the inevitable volume effect and low intrinsic conductivity cause rapid capacity fading of the TMS anode materials. In our work, a high-entropy metal sulfide (HEMS) MnFeCoNiCu–S anode material was obtained by vulcanization and pyrolysis of quinary MOF precursors. Mixing of multiple cations contributes to the diversity of material chemistry and structure. Strong synergies between Mn, Fe, Co, Ni, and Cu establish a steady electronic structure, and high configurational entropy gives the material excellent mechanical strength and excellent stability. Furthermore, the derived carbon matrix can also improve the conductivity and cycling stability of the HEMS. At a current density of 5 A g⁻¹, the HEMS anode can still provide 326.4 mA h g⁻¹ capacity after 7000 cycles, showing long-term sodium storage durability. The synergies of multiple metals and the transfer of multiple electrons ensure excellent sodium storage, which makes the HEMS a favorable candidate for SIB anode materials.

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New concepts

In our work, we proposed a novel synthetic way for a high-entropy metal sulfide (HEMS). Conventionally, high-entropy materials mostly require synthesis conditions of high pressure and high temperature, and have uncontrollability. We synthesized a HEMS MnFeCoNiCu–S by facile vulcanization and pyrolysis of quinary MOF precursors. This method is controllable by adjusting the metal type of the MOF. There is a synergistic effect between Mn, Fe, Co, Ni, and Cu, which establishes a steady electronic structure. In addition, the high configurational entropy makes the material have excellent stability and mechanical strength. Furthermore, the derived carbon matrix can also improve the conductivity of the HEMS. Hence, as an anode material for SIBs, the MnFeCoNiCu–S anode shows excellent sodium-ion storage performance and long-term durability. The cycling stability of the MnFeCoNiCu–S anode is greatly improved compared to the single metal sulfides and MnFeCoNi–S anode. This work provides a facile way to obtain a high-entropy material, which can be a promising anode candidate with high stability for SIBs.

Introduction

In recent years, high-entropy materials have seen very rapid development and become increasingly popular.^1,2 The concept of a high-entropy material is to introduce different elements (five or more) into a single phase lattice, contributing to high configurational entropy.³ At the same time, high-entropy materials exhibit a unique combination of interactions according to the stoichiometric ratio of the introduced elements and the element type. The concept of high entropy was first adapted to alloy materials,⁴ and then to oxides, ceramics, oxyfluorides, etc.^5–7 The four primary factors that affect the structure and properties of high-entropy materials include cocktail effects, slow diffusion, configurational entropy, and lattice distortion. Because of these factors, high-entropy materials can display unexpected properties for a wide range of applications.^8,9 The use of high-entropy materials in the domain of energy storage has become a current research hotspot.^10,11

As important energy storage devices, sodium-ion batteries (SIBs) play big roles in electric vehicles, portable electronics, wearables, and even in aerospace.¹² Anode materials play an important role in the performance of SIBs.^13–15 Transition metal sulfides (TMSs) are often used as anode materials in SIBs.^16,17 Although TMSs have excellent redox reversibility and high theoretical capacities, unavoidable volume effects and low intrinsic conductivity lead to rapid capacity decay in the TMS materials. The emergence of high-entropy materials provides a promising new path for improving volume expansion.^11,18–21 In the last few years, using high-entropy materials as anode materials good performance has been achieved.^11,18,22 Liang et al. obtained multiple ultrafine-sized high-entropy compound anode materials by applying high-pressure field, which showed exceptional sodium-ion storage capacity.²³ Zhao et al. used the entropy-change to synthesize a highly reversible Cu₄MnFeSnGeS₈ anode material for SIBs.¹⁸ However, at present, there are still some challenges in obtaining high-entropy materials with specific compositions. Specially, high-entropy materials mostly require synthesis conditions of high pressure and high temperature, and have uncontrollability.²⁴ Therefore, suitable methods for the synthesis of HEMS are required and need to be developed.²⁵

In our work, a five-metal MOF (MnFeCoNiCu-MOF) was synthesized by a simple hydrothermal method. A HEMS MnFeCoNiCu–S was obtained by a simple vulcanization reaction and pyrolysis of the MnFeCoNiCu-MOF precursor. The strong synergy between Mn, Fe, Co, Ni, and Cu establishes a steady electronic structure. The high configurational entropy makes the material have excellent stability and mechanical strength, and as a result MnFeCoNiCu–S has a stable crystal structure. At the same time, the mixing of multiple cations helps promote chemical and structural diversity, which makes it possible to realize efficient sodium storage in materials. At a current density of 5 A g⁻¹, the MnFeCoNiCu–S anode can still deliver a specific capacity of 326.4 mA h g⁻¹ after 7000 cycles, showing excellent sodium-ion storage performance and long-term durability. The cycling stability of the MnFeCoNiCu–S anode is greatly improved compared to the single metal sulfides and the MnFeCoNi–S anode. This work offers a facile way to obtain a high-entropy material, which can be a promising anode candidate with high stability for SIBs.

Results and discussion

The synthetic scheme of MnFeCoNiCu–S is shown in Fig. S1 (ESI†). Firstly, the MnFeCoNiCu-MOF was obtained by a simple hydrothermal method. The X-ray diffraction (XRD) pattern (Fig. S2, ESI†) shows that the diffraction peaks of the synthesized MnFeCoNiCu-MOF material are consistent with MOF-74.²⁶ The slight deviation of the diffraction peaks of the high-entropy MnFeCoNiCu-MOF is mainly due to the different ionic radii of metal ions. The average ionic radius of the multiple introduced metal ions is smaller than that of the single metal sample, which causes a reduction in the lattice parameters, resulting in shifts of peaks to higher angles. Thus, the shift of the XRD peaks in MnFeCoNiCu-MOF indicates the successful introduction and doping of various metals.²⁷ An inductively coupled plasma-mass spectrometry (ICP-MS) test was performed to determine the content of MnFeCoNiCu-MOF. Weight percentages of Fe, Co, Ni, Mn and Cu in MnFeCoNiCu-MOF are 4.12%, 5.01%, 4.03%, 1.93% and 0.51%, respectively (Table S1, ESI†). Then the single phase HEMS MnFeCoNiCu–S was obtained by vulcanization and pyrolysis of the MnFeCoNiCu-MOF precursor. The weight percentages of Fe, Co, Ni, Mn and Cu are 5.16%, 6.82%, 6.58%, 3.27% and 1.04%, respectively (Fig. S3, ESI†). The XRD pattern (Fig. S5, ESI†) shows that these diffraction peaks of the HEMS are very close to those of Cu₇S₄ (JCPDS No. 23-0958), but there are certain deviations caused by the doping of various metal ions.

The morphology and micro-structure of the obtained samples were analyzed by field emission scanning electron microscopy (FESEM) and transmission electronmicroscopy (TEM). The MnFeCoNiCu-MOF material has a fusiform structure (Fig. 1a). As shown in Fig. 1b and c, MnFeCoNiCu–S obtained after vulcanization also has a homogeneous fusiform structure with a relatively rough surface, and it is clear that this fusiform structure of MnFeCoNiCu–S is composed of nanoparticles. TEM images (Fig. 1d) also display the fusiform structure composed of nanoparticles. In the HRTEM images (Fig. 1e), well-aligned and uniform fringes with lattice spacings of 0.184 nm, 0.254 nm, 0.263 nm and 0.307 nm can be observed, which correspond to the (886), (20, 4, 0), (20, 0, 1) and (804) crystal faces of the HEMS, respectively. The diffraction rings in the selected area electron diffraction (SAED) pattern (Fig. 1f) correspond to the (804) and (886) crystal faces of the HEMS. From high-angle annular dark-field scanning TEM (HAADF-STEM) images (Fig. 1g) and energy-dispersive spectroscopy (EDS) element mapping images, it can be observed that Mn, Fe, Co, Ni, Cu and S elements are homogenously distributed without aggregation and separation. Meanwhile, the FESEM images of the contrast sample MnFeCoNi-MOF (Fig. S6a and b, ESI†) and MnFeCoNi–S (Fig. S6c and d, ESI†) also show the fusiform structure.

Fig. 1 (a) FESEM images of MnFeCoNiCu-MOF. (b) and (c) FESEM images of MnFeCoNiCu–S. (d) and (e) TEM image and HRTEM image of MnFeCoNiCu–S. (f) SAED pattern of MnFeCoNiCu–S. (g) HAADF-STEM image and EDS elemental mapping images of MnFeCoNiCu–S.

Furthermore, the valence and element composition of these elements were examined through X-ray photoelectron spectroscopy (XPS) analyses. It can be detected from the XPS survey spectrum of MnFeCoNiCu–S (Fig. S7, ESI†) that Mn, Fe, Co, Ni and Cu elements are present simultaneously. The high-resolution XPS spectrum of Mn 2p (Fig. 2a) shows that Mn is in the form of Mn²⁺ (641.8 eV, 653.5 eV). Fe is mainly in the valence of 3+ (712.4 eV, 723.5 eV, Fig. 2b). As shown in Fig. 2c, Co exists in the mixed valence state of Co²⁺ (781.9 eV, 798.4 eV) and Co³⁺ (778.8 eV, 793.5 eV). It can be seen from Fig. 2d that Ni exists in mixed valence states of Ni²⁺ (853.7 eV, 872.8 eV), and Ni³⁺ (856.3 eV, 875.5 eV).^28–31 Cu is in the form of Cu⁺ (932.4 eV, 952.0 eV, Fig. 2e). From Fig. 2f, the S element occurs in the state of (S₂)²⁻ (163.9 eV, 164.8 eV) and S²⁻ (161.9 eV, 163.0 eV). The introduction of Cu enhances the intermetallic coupling effect. By comparing the monomeric peak values of MnFeCoNi–S (Fig. S9, ESI†) and MnFeCoNiCu–S (Fig. 2), it is found that the 2p_3/2 peak of Fe, Mn and Ni has an obvious positive shift (Fe > 0.5 eV, Mn > 0.45 eV, Ni > 0.375 eV), and the Co²⁺ peak has a negative shift (−0.33eV). Cu is in a low oxidation state (Cu⁺). These results indicate that there are electron transfers from Fe, Mn and Ni to Co and Cu because of the introduction of Cu, showing a strong coupling effect of these metals, which results in changes in electron density between metals. The interaction between metals can enhance the storage capacity of sodium ions.

Fig. 2 High-resolution XPS spectra of (a) Mn 2p, (b) Fe 2p, (c) Co 2p, (d) Ni 2p, (e) Cu 2p and (f) S 2p of MnFeCoNiCu–S.

The specific surface area plays an important role in the electrochemical sodium storage performance. According to the N₂ absorption/desorption curve (Fig. S10a and c, ESI†), the specific surface areas of MnFeCoNi–S and MnFeCoNiCu–S are 13.47 m² g⁻¹ and 43.50 m² g⁻¹, respectively. These results indicate that the HEMS has a larger specific surface area, and can offer more reaction sites for Na⁺ storage. From the pore size distribution diagram shown in Fig. S10b and d (ESI†), it can be observed that MnFeCoNi–S and MnFeCoNiCu–S have hierarchical porous structures with micropores, mesopores and macropores, mainly mesoporous pores. The MnFeCoNiCu–S sample has a larger pore volume, which is more favorable for electrolyte transport and penetration.

HEMS exhibits excellent electrochemical properties due to its excellent stability and mechanical strength benefiting from strong synergies and high configurational entropy. In the cyclic voltammetry (CV) curve (Fig. 3a) for the MnFeCoNiCu–S anode measured in the first cycle at a scan rate of 0.1 mV s⁻¹, a reduction peak at 0.77 V is observed, which comes from the decomposition of the organic electrolyte resulting in a solid electrolyte interface (SEI) film. Meanwhile, multi-pair redox peaks indicate that redox reactions occur in multiple metals during the process of sodiation and desodiation. The CV curves for the next four cycles greatly coincide, indicating that electrochemical reactions of the high-entropy sulfide electrodes are well reversible. As shown in Fig. 3b, the discharge and charge curves of the initial five cycles at 0.5 A g⁻¹ show that the discharge capacity and charge capacity of the first cycle of the high-entropy sulfide electrode are 587.7 mA h g⁻¹ and 473.6 mA h g⁻¹, respectively. The Coulombic efficiency (CE) in the 1st cycle is 80.6%. The charge and discharge curves for the next four cycles match well, and the CE values are more than 99%, which also demonstrate the excellent reversibility of the anode material. The charge and discharge curves of monometallic sulfides were also tested separately and the results are shown in Fig. S12 (ESI†). The first cycle CE values of these monometallic sulfides are lower than that of the MnFeCoNiCu–S.

Fig. 3 (a) CV curves of the MnFeCoNiCu–S anode at 0.1 mV s⁻¹. (b) Discharge–charge curves of the MnFeCoNiCu–S anode at 0.5 A g⁻¹. (c) Discharge–charge profiles of the MnFeCoNiCu–S anode at different current densities. (d) Cyclic performance of the MnFeCoNiCu–S anode at 0.5 A g⁻¹. (e) Cyclic performance of the MnFeCoNiCu–S and MnFeCoNi–S anode at 5.0 A g⁻¹. (f) Rate performance of MnFeCoNiCu–S and MnFeCoNi–S. (g)–(l) Kinetic analysis of the MnFeCoNiCu–S anode. (g) CV curves in the scan rate range of 0.1–1 mV s⁻¹. (h) Log(i) vs. log(v) plots based on redox peak currents and scan rates. (i) Capacitance and diffusion contribution ratio at altered scan rates. (j) Capacitance contribution ratio at a scan rate of 0.8 mV s⁻¹. (k) EIS spectra of MnFeCoNiCu–S and MnFeCoNi–S and an equivalent circuit model of EIS fitting (inset). (l) The relationship between the real part of the impedance and low frequencies.

Fig. 3d and Fig. S11 (ESI†) show the cyclic performance of the HEMS MnFeCoNiCu–S anode material at current densities of 0.5 and 2.0 A g⁻¹, respectively. After 100 cycles, the corresponding specific capacities are 485.5 mA h g⁻¹ and 409.1 mA h g⁻¹. It can be seen from Fig. 3e that the MnFeCoNi–S anode material reveals a higher specific capacity than the HEMS MnFeCoNiCu–S anode material at a current density of 5 A g⁻¹. However, the specific capacity of the MnFeCoNi–S anode material decreases rapidly after 3580 cycles. The MnFeCoNi–S anode material cannot show long cycling stability. Although the HEMS anode material has lower capacity than the MnFeCoNi–S anode material, the high entropy configuration strategy makes the HEMS anode material show excellent cycling stability and long-term durability, and can still maintain 326.4 mA h g⁻¹ specific capacity with a high capacity retention rate (91.2%) after 7000 cycles. Cycling performances of CoS₂, NiS₂, Cu_1.96S, MnS₂ and Fe₇S₈ anode materials at 5.0 A g⁻¹ are shown in Fig. S13 (ESI†). CoS₂, NiS₂, and Fe₇S₈ displays obvious capacity decay. The specific capacity of MnS₂ is very low and increases, which indicates that the anode is unstable. The specific capacity of Cu_1.96S anode materials decreases to 250.5 mA h g⁻¹ after 2500 cycles. MnFeCoNiCu–S has the highest specific capacity and the longest cycling stability (Table S2, ESI†). In addition, the HEMS MnFeCoNiCu–S anode material exhibits a higher current tolerance and higher rate performance than the MnFeCoNi–S anode material (as shown in Fig. 3c and f). The results also point out that high configurational entropy makes the structure of the HEMS MnFeCoNiCu–S anode material more stable. Furthermore, it can be seen from Fig. S14 (ESI†) and Table S3 (ESI†) that HEMS has better long-term cycling stability than most of the metal sulfides.

Rapid ion transport kinetics determines the outstanding electrochemical performance of anode materials. So, CV curves of MnFeCoNi–S and MnFeCoNiCu–S electrodes at different scan rates of 0.1–1 mV s⁻¹ were obtained, respectively, to measure the storage kinetics of Na⁺ ions (Fig. 3g and Fig. S15a, ESI†). By processing the data of redox peak currents and scan rates, the b values are obtained. As shown in Fig. 3h and Fig. S15b (ESI†), the b values of the five distinct redox peaks in the MnFeCoNiCu–S and MnFeCoNi–S anode materials are close to 1. The redox process for the MnFeCoNiCu–S and MnFeCoNi–S anodes is both dominated by capacitive behavior. In order to calculate the percentages of Na⁺ ion capacitance contribution, the diffusion contribution and surface capacitance contribution at different scan rates are calculated according to the formula (1)–(3):

i = av^b (1)

log(i) = b log(v) + log(a) (2)

i = k₁v + k₂v^1/2 (3)

According to Fig. 3i, j and Fig. S15c, d (ESI†), the MnFeCoNiCu–S and MnFeCoNi–S anodes have a relatively high contribution ratio of capacitance at different scanning rates. When the sweep rate reaches 0.8 mV s⁻¹, the capacitance contributions of MnFeCoNiCu–S and MnFeCoNi–S anodes are 86% and 87%, respectively, indicating that the MnFeCoNiCu–S and MnFeCoNi–S anodes have good kinetic behavior.

The EIS test (Fig. 3k) further demonstrates the good kinetics of the MnFeCoNiCu–S anode. EIS analysis can be used to characterize the dynamics of sodium-ion batteries, and the EIS results are fitted by an equivalent circuit diagram model. R₁, R₂ and CPE represent electrolyte resistance, charge transfer resistance and constant phase element. The R₂ values of the charge transfer resistance of the MnFeCoNiCu–S and MnFeCoNi–S anode materials are 32.61 Ω and 34.61 Ω, respectively. MnFeCoNiCu–S has smaller values of R₁ and R₂ than those of MnFeCoNi–S, suggesting the favorable ion/charge transfer speed and good electrical conductivity of MnFeCoNiCu–S. The line in the low frequency region is Warburg diffusion. Warburg diffusion is essentially produced by ion diffusion. As shown in Fig. 3l, the Weber factor δ of the MnFeCoNiCu–S anode material is smaller than that of the MnFeCoNi–S anode material, indicating that the Na⁺ ion transfer speed is faster in the MnFeCoNiCu–S anode. The EIS is also a vital tool for assessing the diffusion coefficient of Na⁺ in electrode materials. The calculation can be carried out using the formulae (4) and (5):

Z = R_D + R_L + δω^−1/2 (4)

The results of the calculations display that the diffusion coefficient of Na⁺ ions in the MnFeCoNiCu–S anode (6.13 × 10⁻¹¹ cm² s⁻¹) is greater than that of the MnFeCoNi–S anode (3.02 × 10⁻¹¹ cm² s⁻¹), which suggests that the MnFeCoNiCu–S anode has faster kinetic behavior.

The outstanding sodium storage properties and decent reversibility of the HEMS MnFeCoNiCu–S anode materials significantly encourage us to investigate the stability of their electrode structures during cycling. The XRD and FESEM tests on the MnFeCoNi–S anode after cycling (shown in Fig. S17, ESI†) indicate that the crystal structure and morphology of the MnFeCoNi–S anode after cycling have great changes and collapses. Thus, the sharp decrease in the specific capacity of the material MnFeCoNi–S after 3580 cycles is due to damage of the electrode material. It is clear from Fig. S18 (ESI†) that the HEMS MnFeCoNiCu–S anode material after 100, 500 and even 1000 cycles still maintains its original fusiform structure, indicating that the HEMS MnFeCoNiCu–S anode material has excellent structural stability.

The mechanism of sodium storage for the HEMS anode material was explored by XPS (as presented in Fig. 4). It can be observed from Fig. 4 that when the anode material is discharged to 0.77 V, the peaks of Mn²⁺, Co²⁺ and Ni³⁺ have obvious negative shifts, indicating the conversion of Mn²⁺ to Mn⁰, Co³⁺ to Co, and Ni³⁺ to Ni^2–3+. Meanwhile, there is a little negative shift of the Fe³⁺ peak, which is because of the conversion of Fe³⁺ to Fe. There is no shift of the Cu⁺ peak. When charged to 2.8 V, Mn is re-transformed into Mn²⁺, Co is transformed into Co²⁺, Ni^2–3+ is re-transformed into Ni²⁺ and Ni³⁺, and Fe is re-transformed into Fe³⁺, indicating good reversibility of the MnFeCoNiCu–S anode.

Fig. 4 Ex suit XPS spectra of (a) Mn 2p, (b) Co 2p, (c) Ni 2p, (d) Cu 2p, (e) Fe 2p and (f) corresponding charge and discharge curves.

Furthermore, the sodium storage mechanism of the HEMS anode was studied by density functional theory (DFT) calculations (as displayed in Fig. 5). The (804) plane is chosen as the model. The optimized structures of the MnFeCoNiCu–S and contrast sample Cu₇S₄ are the calculation model (presented in Fig. 5a and b). The schematic diagrams of the Na adsorption sites on the MnFeCoNiCu–S and contrast sample Cu₇S₄ are presented in Fig. 5a and b. The Na adsorption energies of the MnFeCoNiCu–S and contrast sample Cu₇S₄ (Fig. 5c) are −2.85 eV and −2.48 eV, respectively. The larger Na adsorption energy of MnFeCoNiCu–S suggests the enhancement of adsorption of Na⁺ to boost the electrochemical reaction. The density of states (DOS) of the MnFeCoNiCu–S and contrast sample Cu₇S₄ (Fig. 5d and e) show that MnFeCoNiCu–S has stronger density of states and more spin orbitals than those of the contrast sample Cu₇S₄ near the Fermi level, which suggests that MnFeCoNiCu–S has higher conductivity. The Na⁺ ion transfer energy barriers of Cu₇S₄ (Fig. 5f), (MnFeCoNi)S₂ (Fig. S21, ESI†) and (MnFeCoNiCu)₇S₄ (Fig. 5g) show that (MnFeCoNiCu)₇S₄ has the lowest Na⁺ ion transfer energy barriers. The result indicates that the high-entropy effect can accelerate the transfer of Na⁺ ions. The DFT results indicate that MnFeCoNiCu–S has strong adsorption of Na⁺ ions, enhanced conductivity and low Na⁺ ion transfer energy barriers, which come from the high-entropy effect. These advantages endow (MnFeCoNiCu)₇S₄ with excellent electrochemical performance.

Fig. 5 The adsorption model for Na⁺ ions (a) Cu₇S₄ and (b) (MnFeCoNiCu)₇S₄. (c) The adsorption energy for Na⁺ ions of Cu₇S₄ and (MnFeCoNiCu)₇S₄, DOS for (d) Cu₇S₄ and (e) (MnFeCoNiCu)₇S₄. Na⁺ ion diffusion paths and energy barriers in (f) Cu₇S₄ and (g) (MnFeCoNiCu)₇S₄.

The HEMS MnFeCoNiCu–S has demonstrated admirable sodium storage performance as an anode material for half cells, which greatly attracted us to measure its performance in full batteries. As presented in Fig. 6a, MnFeCoNiCu–S and Na₃V₂(PO₄)₃ are used as the anode and cathode in the full battery, respectively. From the charge and discharge curves of the initial 5 cycles of the full battery (Fig. 6b), it can be seen that the assembled full battery has good cycling stability. At a current density of 0.5 A g⁻¹, after 500 cycles, the specific capacity can still remain 260.5 mA h g⁻¹ (Fig. 6c). At the same time, the MnFeCoNiCu–S//Na₃V₂(PO₄)₃ full battery shows outstanding rate performance with specific capacities of 389.8, 319.9, 275.7, 224.5 and 172.7 mA h g⁻¹ at a series of current densities ranging from 0.1 to 2 A g⁻¹, respectively (Fig. 6d). Once the current density returns to 0.2 A g⁻¹, the specific capacity of the full battery can be restored to 322.1 mA h g⁻¹, showing high current tolerance and good reversibility. The charge–discharge curves and the cycle performance of the MnFeCoNi–S//Na₃V₂(PO₄)₃ full battery were also measured, which are lower than those of the MnFeCoNiCu–S Na₃V₂(PO₄)₃ full battery (Fig. S22 and Table S4, ESI†). These results indicate that the high-entropy effect makes the material show a better full battery performance.

Fig. 6 (a) Schematic diagram of the MnFeCoNiCu–S//Na₃V₂(PO₄)₃ full battery. (b) Discharge–charge curves of the full battery at 0.5 A g⁻¹. (c) Cycling performance at 0.5 A g⁻¹. (d) Rate performance of the full battery.

Conclusions

A HEMS MnFeCoNiCu–S was synthesized by a simple vulcanization reaction of a five-metal MOF precursor. Experimental tests and DFT prove that the high configurational entropy gives the material excellent stability and excellent mechanical strength, and the strong synergies between Mn, Fe, Co, Ni and Cu establish a stable electronic structure. The synergies of multiple metals and multiple electron transfers ensure excellent sodium storage capability. At a current density of 5 A g⁻¹, the HEMS MnFeCoNiCu–S as a SIB anode material can still offer 326.4 mA h g⁻¹ reversible specific capacity after 7000 cycles, which significantly improves the cycling stability of the electrode material.

Author contributions

Wei Shuang: conceptualization, data curation, methodology, formal analysis, writing – original draft, and funding acquisition. Junjia Xu: methodology, investigation, visualization, and formal analysis. Fuyou Chen: investigation and formal analysis. Yujun Wu: funding acquisition and formal analysis. Lin Yang: resources, supervision, and writing – review and editing. Zhengyu Bai: methodology, project administration, resources, supervision, validation, writing – review and editing, and funding acquisition.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

All authors greatly acknowledge that the work was financially supported by the National Natural Science Foundation of China (Grant No. 22305071 and 52472200), the 111 Project (Grant No. D17007), China Postdoctoral Science Foundation (Grant No. 2022M721049), Natural Science Foundation of Henan Province (Grant No. 252300421556), Henan Center for Outstanding Overseas Scientists (Grant No. GZS2022017), and Henan Province Key Research and Development Project (Grant No. 231111520500).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5mh00163c

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