Construction of Highly Active Co₃S₄/Fe₇S₈ Heterostructures Derived from Sodium Alginate for Enhanced Sodium Storage Performance

Abstract

Heterointerface engineering, especially the construction of heterointerfaces based on two highly active components, is an effective strategy to enhance the sodium storage capacity and accelerate the reaction kinetics of transition metal chalcogenide anodes. Herein, a series of SA-CoFe-S composites composed of two highly active metal sulfides, Co₃S₄ and Fe₇S₈, were fabricated through in situ chelation effects coupled with a one-step sulfurization strategy. The optimized SA-CoFe(1:4)-S is composed of fine nanoparticles encapsulated by uniformly distributed S-doped carbon. This unique carbon confinement effect and nano-sized active particles can alleviate volume expansion, shorten the ion diffusion distance, and accelerate electron transfer. In addition, the strong electric-field effect and rich heterointerfaces generated by the heterostructure provide more active sites for sodium storage and accelerate the sodium storage kinetics. The relevant theoretical calculation outcomes further confirm that the heterointerfaces formed between Co₃S₄ and Fe₇S₈ can enhance the adsorption energy toward sodium ions and boost the electrical conductivity of the composite material. As an anode material for sodium-ion batteries, the initial discharge/charge capacities were 723/1010 mAh·g⁻¹, exhibited at 1 A·g⁻¹, and the coulombic efficiency (CE) corresponding to this current density was measured to be 71.6%. Even after 800 cycles, the reversible discharge specific capacity of the electrode can still reach 806 mAh·g⁻¹ at 1 A·g⁻¹. Additionally, at an elevated current density of 3 A·g⁻¹, the electrode sustains stable cycling over 500 cycles, with its discharge capacity kept at 258 mAh·g⁻¹ after the long-term cycling test.

1. Introduction

Similar working principles, abundant natural reserves, and low costs make sodium-ion batteries (SIBs) stand out among many secondary batteries and become the best substitute for lithium-ion batteries (LIBs) [1,2,3]. Currently, the energy density of sodium-ion batteries is primarily restricted by their anode materials. The development of negative electrode materials with a high specific capacity and excellent rate performance is essential for improving the energy density of sodium ion batteries [4,5,6]. As the ionic radius of Na⁺ (1.02 Å) is substantially greater than that of Li⁺ (0.76 Å), pronounced volumetric deformation is caused in anode materials during the charge–discharge cycling of sodium-ion batteries, with the migration kinetics of Na⁺ being impaired accordingly [7,8,9]. This makes it difficult for many anode materials that perform excellently in LIBs to play their roles in SIBs. Therefore, designing and preparing high-capacity anode materials suitable for SIBs remains an important challenge at present.

Metal sulfides, as the promising anode, possess a larger lattice spacing and lower migration barriers than metal oxides [10,11,12]. Benefitting from these advantages, they are more suitable as anode materials for SIBs. This is particularly evident in Fe-based sulfides (such as FeS, FeS₂, and Fe₇S₈), which have received extensive attention due to their excellent theoretical capacity [13,14,15]. Among them, Fe₇S₈ exhibits mixed-valence Fe elements, inherent metallic properties, and a high theoretical capacity (663 mAh·g⁻¹). Metal sulfides, regarded as potential anode materials, exhibit a larger lattice spacing and lower migration energy barriers than metal oxides [10,11,12]. Owing to these merits, they are more adequately suited for use as anode materials in SIBs. Such suitability is especially noticeable in Fe-based sulfides (including FeS, FeS₂, and Fe₇S₈), as extensive attention has been paid to them due to their outstanding theoretical capacity [13,14,15]. Specifically, Fe₇S₈ is endowed with mixed-valence Fe species, inherent metallic properties, and a high theoretical capacity of 663 mAh·g⁻¹ [16,17]. However, Fe₇S₈, which is based on the conversion mechanism, undergoes significant volume expansion during the intercalation of Na⁺, resulting in agglomeration and pulverization, thereby exacerbating the capacity decay. Moreover, it still delivers a poor electrical conductivity, which reduces its electron mobility and rate capability [18,19]. Therefore, stabilizing the cycle performance of Fe₇S₈ while improving its rate performance to achieve fast charging and discharging capabilities is crucial for practical applications. To circumvent these issues, lots of strategies, such as the modification of conductive carbon materials (graphene, carbon nanotube, and polymer-derived carbon), the design of nanostructures, the introduction of multiple components, and electrostructure engineering, have been proposed [20,21,22,23,24,25]. Among them, the incorporation of another phase can form a heterostructure with the Fe₇S₈, creating abundant heterointerfaces and providing more active sites for sodium storage, thereby enhancing the material’s capacity. Furthermore, the difference in Fermi levels between the two phases enables the directional flow of electrons at the interface, forming a built-in electric field. This not only boosts the material’s conductivity but also achieves an excellent rate performance. For example, Wen et al. [26] constructed an Fe₇S₈/FeS₂ heterostructure to boost reaction kinetics for superior fast sodium storage. The optimized NHCFs-Fe₇S₈/FeS₂ exhibited an outstanding long-term stability at 1 A·g⁻¹ (596 mAh·g⁻¹ after 1000 cycles) and rate capability (155 mAh·g⁻¹ at 50 A·g⁻¹). Yin et al. [27] rationally designed an Fe₇S₈/FeP@NC heterostructure through a one-step carbonization, phosphating, and vulcanization method. This unique heterogeneous structure can provide abundant active sites and shorten the Na⁺ transport paths, thereby effectively optimizing reaction kinetics and stability [28,29,30]. Based on these results, constructing heterostructures is highly effective for attempting to enhance Fe₇S₈ capacity while maintaining cyclic stability and high-rate performance.

In this paper, we fabricated a unique Fe₇S₈/Co₃S₄ heterostructure using a facile in situ chelation effect coupled with the sulfurization strategy. Notably, the abundant hydroxyl (−OH) and carboxyl groups (−COOH) in sodium alginate can provide core sites for metal ion capture. Meanwhile, coming from the difference in chelating ability between the two metal ions and the functional groups, the content of the two phases in the product can be regulated by varying the molar ratio of the metal ions. The characterization results confirm that the optimized product SA-CoFe(1:4)-S, with a cobalt-iron molar ratio of 1:4, contains abundant Co₃S₄ and Fe₇S₈. The coexistence of two phases enables the generation of a highly active heterostructure. The Fe₇S₈/Co₃S₄ heterostructure constructs fast ion transport channels between heterointerfaces, which effectively enhances Na⁺ diffusion efficiency and optimizes electrochemical reaction sites. Moreover, its uniformly dispersed small nanoparticles not only supply abundant active sites but also alleviate the volume expansion for the material. Owing to these positive effects mentioned above, SA-CoFe(1:4)-S can deliver a high initial discharge capacity of 1010 mAh·g⁻¹ at 1 A·g⁻¹ and an exceptional cycle performance (273 mAh·g⁻¹ retained after 600 cycles at 3 A·g⁻¹). This work lays a foundation for the design and preparation of high-performance iron-based sulfide heterostructures.

2. Experimental Methods

2.1. Materials

Co(NO₃)₂·6H₂O, Fe(NO₃)₃·9H₂O and S powder were obtained from Aladdin Chemical Co., Ltd (located in Shanghai, China). Meanwhile, the sodium alginate was purchased from China Pharmaceutical Chemicals Co., Ltd. (located in Shanghai, China) Notably, all the reagents were analytically pure and without further purification.

2.2. Synthetic Methods

First, 3 g of sodium alginate (SA) was dissolved in 100 mL of H₂O, and, after magnetic stirring for 1 h, solution A was obtained. Subsequently, 1.63 g of Co(NO₃)₂·6H₂O and 9.049 g of Fe(NO₃)₃·9H₂O (the molar ratio of Co to Fe is 1:4) were successively dissolved in 200 mL of water, and solution B was obtained. Then, solution A was dropped into solution B. The obtained mixed solution was placed in a water bath at 30 °C and left standing for 24 h. After that, it was washed three times with deionized water and freeze-dried for 48 h to obtain the SA-CoFe(1:4) precursor. Finally, 600 mg of the precursor (downstream) and 3 g of sulfur powder (upstream) were put into a ceramic crucible. Under an Ar gas atmosphere, the tube furnace was heated up to 600 °C at a rate of 2 °C min⁻¹ and kept at the temperature for 4 h; after this, it naturally cooled down to room temperature. The achieved black powder was denoted as SA-CoFe(1:4)-S. For the contrast materials, since the total concentration of metal salts remains unchanged, only the molar ratios of cobalt to iron were adjusted to 15:1, 1:1, 4:1, and 1:15. The resulting products were labeled as SA-CoFe(15:1)-S, SA-CoFe(1:1)-S, SA-CoFe(4:1)-S, and SA-CoFe(1:15)-S, respectively. Meanwhile, the products obtained by adding only one type of metal salt and without metal salt were labeled as SA-Co-S, SA-Fe-S, and SA-S.

2.3. Materials Characterization

The crystal phase of materials was investigated using X-ray diffraction (XRD, Bruker AXS D8 Advance is from the German BRUKER-AXS company located in Karlsruhe, Germany, Cu Kα, λ = 0.154 06 nm) under 40 kV and 40 mA; the scanning range was 10° to 90°, and the step size was 0.022°. Simultaneously, scanning electron microscopy (SEM, FEI Quanta FEG 250 is from FEI Company in Hillsboro, OR, USA, 5.0 kV) was performed to explore the microstructure and morphology of the synthesized materials. The element valence states and elemental composition of the materials were also analyzed using an Escalab 250Xi X-ray photoelectron spectrometer (which is from Thermo Fisher Scientific, a company located in Waltham, MA, USA, Al Kα, 1 486.6 eV). Finally, the metal content of all materials was determined using ICP-OES (Agilent ICP-OES 725 ES is from Shanghai Aiyin Precision Instrument Technology Co., Ltd. located in Shanghai, China).

2.4. Electrochemical Measurements

Electrochemical performance was further evaluated by assembling coin half-cells. In particular, the working electrodes were constructed by grounding the obtained material (80%), conductive agent (Super P, 10%), and CMC (Carboxymethyl cellulose, 10%). The loading weight of the material was about 1.0 mg cm⁻². In addition, the counter electrode and electrolyte were Na metal and 1 M NaPF₆ in DEGLYME, respectively. The corresponding separator was the Whatman glass fibers (GF/D). The Galvanostatic charge/discharge (GCD) tests, long cycling performance, and rate performance were achieved using a Neware multichannel battery test system from Wuhan Kingnuo Electronic Co. (Wuhan, China). The voltage window for half-cell testing was 0.01–3.0 V (vs. Na⁺/Na). Finally, the cyclic voltammetry (CV) curves and Electrochemical impedance spectra (EIS, frequency range 0.01–10⁵ HZ, amplitude was 5 mV) were obtained using a DH7000D electrochemical workstation (Jiangsu Donghua Analytical Instrument Co., Ltd., Taizhou, China).

3. Results and Discussion

Figure 1a illustrates the preparation procedure of the SA-CoFe-S material. As we know, sodium alginate (SA) is composed of β-D-mannuronic acid (M unit) and α-L-guluronic acid (G unit) and exhibits viscosity when dissolved in water [31]. The increase in viscosity is mainly attributed to the physical self-crosslinking process, where polar groups such as –OH and –COOH groups on the sodium alginate molecular chains form intermolecular hydrogen bonds with those on adjacent molecular chains. Excitingly, the Na⁺ in the G units of its structure can be easily replaced by various metal ions (Co²⁺, Cu²⁺, Fe³⁺, and Ca²⁺), forming hydrogel spheres with a unique “egg-shell” structure [12,32]. Leveraging this metal ion-capturing capability, the SA solution was dropped into a mixed solution containing Co²⁺/Fe³⁺ to form SA-CoFe hydrogel spheres, as shown in Figure S1. After reaction for 24 h, the loaded Co²⁺/Fe³⁺ hydrogel spheres were rinsed with clean water to remove physically adsorbed Co²⁺ and Fe³⁺ on the surface. Subsequently, the obtained SA-CoFe hydrogel spheres were freeze-dried to form yellow products. Finally, the freeze-dried SA-CoFe products were converted into SA-CoFe-S, which contains a highly active Co₃S₄/Fe₇S₈ heterostructure, via a one-step sulfidation treatment.

To investigate the morphology change in SA, the corresponding SEM images were obtained. It can be seen that the initial SA exhibited a curved micro-rod structure (Figure S2a–c). However, SA-Co and SA-Fe, obtained after adsorbing metal ions and undergoing freeze-drying treatment, exhibit morphologies significantly different from that of pristine SA. It can be seen in Figure S3 that SA-Co presents a blocky structure with a rough surface, while SA-Fe shows an irregular structure with a porous network. Such morphological differences indicate that the microstructure of sodium alginate has changed after physical self-crosslinking and in situ chelation with metal ions. In addition, the results also show that the hydrogel spheres formed via chelation reactions exhibit differences in micromorphology depending on the type of metal salt. To obtain metal sulfides with a high sodium storage activity, the SA-Co, SA-Fe, and SA-CoFe(1:4) precursors were further in situ converted into SA-Co-S, SA-Fe-S, and SA-CoFe(1:4)-S through sulfidation treatment. As depicted in Figure 1b,c, the obtained SA-Co-S still maintains a large bulk structure with fine particles distributed on its surface. Relevant EDS element analysis (Figure 1d) shows that Co, S, and C elements are present in the material. Turning to SA-Fe-S (Figure 1e–g), its porous interconnected network structure converts into granular iron-sulfide formations, where Fe, S, and C elements are evenly spread throughout the sample. Different from the previous two materials, the SA-CoFe precursor exhibits a small-sized nanoparticle morphology after sulfidation (Figure 1h,i). Such tiny active nanoparticles are capable of revealing additional active sites, reducing ion diffusion paths, and mitigating volume expansion in sodium storage procedures [33]. Moreover, elemental examination further verifies the co-presence of Co, Fe, and S in SA-CoFe(1:4)-S (Figure 1j), where Fe exhibits a more focused distribution. This result also indicates the successful preparation of bimetallic sulfides, in which iron-based sulfides dominate the material. To enable a comparative study, the resulting SA-S product was additionally subjected to the characterization and analysis of its micro-morphology, with the findings illustrated in Figure S2d–f. The findings revealed that the structural features of the SA-S product, obtained via the self-crosslinking and sulfidation of dissolved sodium alginate in water, underwent a significant transformation, exhibiting a large nanosheet structure.

The TEM and high-resolution TEM images can further verify the morphologies of all samples. Figure 2a shows that the obtained SA-Co-S has a block structure. Meanwhile, HR-TEM images show that the interplanar spacings (d-spacing) of 0.332, 0.235, and 0.285 nm are assigned to the (220), (400), and (311) planes of Co₃S₄ (PDF#42-1448), respectively [34]. For SA-Fe-S (Figure 2b), the block structure can still be observed. Furthermore, the d-spacing of 0.297/0.298 and 0.206 nm is indexed to the (200) and (206) planes of Fe₇S₈ (PDF#24-0220), respectively [35]. In Figure 2c, the d-spacing of 0.333 and 0.285 nm is assigned to the (220) and (311) planes of Co₃S₄. Simultaneously, the d-spacing of 0.297 nm is assigned to the (200) plane of Fe₇S₈. Furthermore, the interface between the (311) plane of Co₃S₄ and the (200) plane of Fe₇S₈ can be observed, indicating the formation of a phase heterointerface. The carbon layer derived from the SA skeleton wraps around the active particles, which can alleviate the volume expansion. From Figure 2d, the Fe, Co, and S elements can be detected in SA-CoFe(1:4)-S. Compared to the Co element, the distribution of Fe is denser, which suggests that the content of Fe₇S₈ in the material is higher.

The crystal structure of SA-CoFe(1:4)-S and other materials were analyzed using XRD, as depicted in Figure 3a. It can be seen that the characteristic peaks of materials with different molar ratios of cobalt and iron salts correspond to the standard Co₃S₄ (PDF No.42-1448) [34] and Fe₇S₈ (PDF No.24-0220) [35], respectively. For SA-CoFe(1:4)-S, the peaks located at 31.4°, 38.0°, and 55.0° correspond to the (311), (400), and (440) crystal planes of Co₃S₄, respectively. Simultaneously, the diffraction peaks located at 30.2°, 34.0°, 44.1°, and 53.5° correspond to the (200), (203), (206), and (305) crystal planes of Fe₇S₈. This result further confirms the coexistence of the Co₃S₄ and Fe₇S₈ phases, which is consistent with the TEM results. The lattice parameters of the Co₃S₄ phase calculated based on the lattice spacing data obtained from TEM are a = b = c = 0.941 nm, while the lattice parameters of the Fe₇S₈ phase are a = b = 0.687 nm, c = 1.713 nm. The lattice parameters of Co₃S₄ calculated from XRD scattering data are a = b = c = 0.945 nm, and those of Fe7S8 are a = b = 0.687 nm, c = 1.705 nm. The results indicate that the lattice phase of Co₃S₄ slightly shrinks, while the Fe7S8 phase shows a slight increase. The reduction in Co₃S4 increases the defect sites and Na⁺ channels, thereby enhancing adsorption/intrusion and reducing diffusion barriers, while the expanded Fe₇S₈ at the interface reduces the Na⁺ diffusion obstacles and promotes rapid insertion/deletion. Moreover, this change can alleviate internal stress, inhibit electrode volume expansion, and improve structural stability and cycle durability. In particular, the diffraction peak of 29.3° can be assigned to the (200) plane of FeS. It is worth noting that, with the introduction of Co, the crystallinity of the FeS phase gradually increases, while the crystallinity of Fe₇S₈ gradually decreases. The dynamic regulation of cobalt ions not only leads to the formation of a multi-component heterogeneous structure consisting of Co₃S₄, Fe₇S₈, and disordered FeS, which supplies a rich heterointerface and enhances the built-in electric field effect to accelerate the migration of sodium ions, but also retains a large number of lattice defects, dislocations, and unsaturated bonds in the disordered FeS phase, providing additional active sites for Na⁺ storage. Interestingly, by comparing the XRD patterns of all prepared materials, it can be observed that, as the Co/Fe molar ratio increases, the diffraction peaks of Co₃S₄ gradually intensify, while those of Fe₇S₈ weaken progressively. This regular variation in diffraction peaks reveals that the contents of Co₃S₄ and Fe₇S₈ in the material change accordingly with the adjustment of the molar ratio. In addition, the product obtained through the direct sulfidation of freeze-dried sodium alginate was also characterized using XRD. Figure 3b demonstrates that the XRD spectrum of SA-S exhibits two distinct diffraction peaks within the angular ranges of 22–24° and 43–44°, and these peaks correspond to the (002) and (100) crystallographic planes of carbon. Derived primarily from the carbon framework of sodium alginate, the resulting carbonaceous substance contributes to enhancing the material’s electrical conductivity. To further confirm the variation in metal content in the products, ICP-OES tests were conducted, as shown in Figure 3c. It can be seen that, as the Co/Fe molar ratio decreases, the Fe content increases gradually. Additionally, after the molar ratio reaches 1:4, the Fe content changes slightly, indicating that the Fe³⁺ chelatable by sodium alginate has reached saturation. Meanwhile, it is obvious that when the molar ratio is 1:4, the Fe content is much higher than that of Co, suggesting that the functional groups in sodium alginate have a stronger capture ability for Fe³⁺. Notably, it is markedly greater than that of cobalt, which suggests that Fe₇S₈ is the primary component of the material. This result aligns well with the outcomes of the EDS and XRD results.

XPS analysis was employed to examine the surface properties and interfacial interplay of the Co₃S₄/Fe₇S₈ heterostructure. The XPS survey of all samples can be observed, as shown in Figure S5. For the Co 2p of SA-CoFe(1:4)-S (Figure 3d), the weak peaks at 782.6, 786.5, and 791.2 eV correspond to Co³⁺, Co²⁺, and satellite peak [36,37]. Compared to the Co₃S₄, the marked reduction in peak intensity stems from the low abundance of Co₃S₄ components within the sample. For SA-Co-S, the additional peak of Co⁰ (778.5 eV) can be detected, indicating that excess cobalt cannot be fully converted into the corresponding sulfide [38]. For Fe 2p (Figure 3e), the peaks at 707.2, 711.2/724.2, 713.7/726.5, and 718.3/731.3 eV correspond to Fe⁰, Fe²⁺, Fe³⁺, and the satellite peak, respectively [39,40,41,42]. Compared to the SA-Co-S and SA-Fe-S, the Co 2p and Fe 2p spectra of SA-CoFe(1:4)-S exhibit a shift towards higher binding energies, a phenomenon resulting from the mutual interaction between the two constituents. Specifically, this shift is mainly due to the flow of electrons from Fe₇S₈ to Co₃S₄ in heterostructures, resulting in an increase in the valence state of Fe and a decrease in the valence state of Co, ultimately leading to a shift in the binding energy. In the case of S 2p spectroscopy (Figure 3f), the peak at 165.1 eV is assigned to the C−S bond, indicating the presence of S-doped carbon; this substance can further supply extra active sites and improve conductivity [43,44,45,46]. Furthermore, the remaining peaks are ascribed to S²⁻ and SO_x. Meanwhile, the S 2p spectra exhibit a slight shift toward a lower binding energy, which is mainly attributed to the charge redistribution and electron density of state modulation at the heterointerfaces. These effects increase the electron cloud density around S atoms and reduce their effective nuclear charge, thereby resulting in a lower binding energy detected by XPS. Consistent with expectations, a peak corresponding to the C−S bond is likewise detected in the C 1s spectrum (Figure 3g). Based on these XPS analyses, they further provide evidence for the bonding state and chemical composition of SA-CoFe(1:4)-S, highlighting the electronic interaction of the phase heterointerfaces derived from the Co₃S₄ and Fe₇S₈ phases. This is crucial for optimizing the electrochemical activity of heterointerfaces.

To further validate the advantages of heterointerfaces in SA-CoFe(1:4)-S, the corresponding electrochemical performance in SIBs was evaluated. Notably, the CV curves were tested at a scanning rate of 0.2 mV·s⁻¹ within a voltage window of 0.01 to 3.0 V. For SA-Co-S (Figure 4a), the reduction peak at 0.02–0.35 V during the first discharge process corresponds to the formation of the SEI film and the conversion reaction of the amorphous Co₃S₄ phase. This distinct peak disappears in the subsequent cycles, while the peaks at 0.51 and 0.77 V are derived from the Na⁺ intercalation process of Co₃S₄. Simultaneously, the peaks at 1.76 and 2.01 V in both the first cycle and subsequent cycles originate from the reversible Na⁺ deintercalation process. For SA-Fe-S (Figure 4b), the reduction peaks at 0.56 V and 0.82/0.33V in the first discharge process correspond to the formation of SEI film and the typical conversion reaction (Fe₇S₈ is converted into Na₂S and Fe). In the first charge process, the oxidation peaks of 1.44 and 1.85 V can be indexed to the desodization process. After the second cycle, the oxidation–reduction peaks overlapped well. As shown in Figure 4c, during the first cathodic scan of SA-CoFe(1:4)-S, the reduction peaks at 0.28, 0.55, and 0.82 V correspond to the formation of the SEI film and the process of Na⁺ intercalating into Co₃S₄ and Fe₇S_8, and being further reduced to metallic Co and Fe [47,48]. During the later cycles, the reduction signal at 0.55 V fails to be detected, which implies the development of a steady SEI layer. At the same time, the reduction peak shifts progressively to 1.22 and 1.53 V. During the anodic sweep, the oxidation peaks located at 1.56 and 2.11 V correspond to the generation of Co₃S₄ and Fe₇S₈ [49,50]. In later cycles, all the redox peaks coincide well, proving that the material can stably undergo redox reactions and has a good reversibility. By comparing the CV curves, it can be clearly observed that SA-CoFe(1:4)-S exhibits the redox peaks of both Co₃S₄ and Fe₇S₈, indicating the coexistence of the two phases. The GCD curves of three materials (Figure 4d–f) at 1 A·g⁻¹ show the voltage plateaus corresponding to the CV curves. Simultaneously, the optimized SA-CoFe(1:4)-S shows the initial discharge/charge specific capacity of 1010/723 mAh·g⁻¹, which is higher than that of SA-Co-S (771/566 mAh·g⁻¹) and SA-Fe-S (527/410 mAh·g⁻¹). The initial coulombic efficiency of SA-CoFe(1:4)-S can reach 71.6%. This elevated reversible capacity is primarily attributed to the cooperative interaction between Co₃S₄ and Fe₇S₈, which can offer extra sodium storage locations, and the intense electric field phenomenon formed at the heterointerfaces facilitates electron transport and ion migration.

For all prepared samples, their cycling stability was further investigated. Figure 5a shows that, after 800 cycles, SA-CoFe(1:4)-S still holds a large capacity of 806 mAh·g⁻¹, exceeding that of SA-Co-S (650 mAh·g⁻¹ after 674 cycles) and SA-Fe-S (386 mAh·g⁻¹ after 800 cycles). The corresponding GCD curves at different cycle numbers can be obtained, as displayed in Figure 5c–e. The SA-CoFe(1:4)-S shows a slower capacity decay trend than the others, revealing the outstanding cyclic stability. To further confirm that 1:4 is the optimal molar ratio, the cycle performance of samples obtained with different molar ratios was further evaluated, as shown in Figure 5b. The evaluation findings indicate that the SA-CoFe(1:4)-S also possesses the best cycling performance. Scanning electron microscopy (SEM) observations (Figure S4) reveal that, after 100 cycles at 1 A·g⁻¹, the prepared SA-Co-S, SA-Fe-S, and SA-CoFe(1:4)-S still exhibit a complete morphology without severe pulverization, demonstrating the excellent structural stability of the prepared material. To further verify the high-rate performance of the SA-CoFe(1:4)-S composites with the optimal molar ratio and the positive effect of the heterostructure, a long-cycle stability test was conducted under a high current density (3 A·g⁻¹). As displayed in Figure 5f, the optimized SA-CoFe(1:4)-S maintains a high reversible capacity (258 mAh·g⁻¹) and coulombic efficiency after 500 cycles, which are superior to those of SA-Co-S (180 mAh·g⁻¹) and SA-Fe-S (188 mAh·g⁻¹). Notably, the cyclic performance of SA-CoFe(1:4)-S is superior to some reported Co- or Fe-based electrode materials, as shown in Table S1 [17,19,26,27,37,38,39,40,41,42,51]. It demonstrates that the heterostructure constructed based on the two highly active components can maximize the advantages of the heterointerfaces and achieve an overall enhancement in sodium storage performance.

To further confirm the facilitating effect of the strong electric field induced by the Co₃S₄/Fe₇S₈ heterostructure on enhancing the material’s electrical conductivity, the charge transfer impedance before and after 20 cycles under 1 A·g⁻¹ was tested, as presented in Figure 6a,b. Before the cycle, SA-Co-S, SA-Fe-S, and SA-CoFe(1:4)-S exhibit a similar charge transfer impedance. However, after 20 cycles, the SA-CoFe(1:4)-S electrode presents a lower charge transfer impedance (4.5 Ω) in the high-frequency region compared to SA-Co-S (5.5 Ω) and SA-Fe-S (8.2 Ω). Additionally, compared with the SA-Co-S and SA-Fe-S electrodes, the SA-CoFe(1:4)-S electrode exhibits a smaller slope in the low-frequency region. These results demonstrate that the heterostructure constructed with Co₃S₄/Fe₇S₈ in the SA-CoFe(1:4)-S composite has an appropriate charge transfer resistance and a larger sodium ion diffusion coefficient, which can achieve a higher specific capacity while ensuring a certain charge–discharge rate. This result further confirms that the construction of a Co₃S₄/Fe₇S₈ heterostructure can significantly enhance the electrical conductivity and cycle stability of electrode materials. Figure 6c,d present the experimental findings regarding the rate capability of samples with varying molar proportions of cobalt and iron salts. It can be seen that the SA-CoFe(1:4)-S material not only has a high capacity but also displays an excellent rate performance. At current densities of 0.1, 0.2, 0.5, 1, and 2 A·g⁻¹, the corresponding specific capacities were 484, 452, 405, 371, and 323 mAh·g⁻¹, respectively. Under a high current density, the SA-CoFe(1:4)-S electrode can also exhibit a high charge–discharge capacity. When the current is restored to 0.1 A·g⁻¹, the capacity recovered to 458 mAh·g⁻¹, indicating a good reversibility and excellent rate performance in the SA-CoFe(1:4)-S electrode. The corresponding GCD curves under different current densities can be observed, as shown in Figure 6e–g. The results show that SA-CoFe(1:4)-S exhibits a smaller capacity decay and excellent capacity retention at different current densities, indicating that the optimized Co/Fe molar ratio and the construction of heterostructure enable the material to achieve a rapid electron transfer rate. Consequently, it obtains an outstanding rate performance and enhances the material’s potential application value.

Figure 6h shows the GITT curves of SA-Co-S, SA-Fe-S, and SA-CoFe(1:4)-S at 0.05 A·g⁻¹. As a major indicator for evaluating sodium storage kinetics, the Na⁺ diffusion coefficient (D_Na+) can be calculated using Fick’s second law [51]. Compared to the values of D_Na+ (Figure 6i,j), it is found that the SA-CoFe(1:4)-S composite delivers the highest value among the others. This result indicates that phase heterointerfaces promote Na⁺ diffusion due to the strong electric field effect, and the formation of built-in electric fields can further achieve efficient electron transfer within materials and at interfaces. Hence, the design of a heterostructure can achieve a superior high-rate cyclin performance and rate capability.

To further elucidate the pivotal roles of heterostructure with the SA-CoFe(1:4)-S anode, Density Functional Theory (DFT) calculations were performed [52,53,54,55]. In Figure 7a, axes a, b, and c correspond to the default visualization directions along the X-axis, Y-axis, and Z-axis of the display interface, respectively. Figure 7a shows the optimized structural models of Co₃S₄, Fe₇S_8, and Co₃S₄/Fe₇S₈. In addition, the total density of states (TDOS) in Figure 7b indicates that Co₃S₄ and Fe₇S₈ all display semiconductor characteristics. However, the Co₃S₄/Fe₇S₈ heterostructure presented a higher carrier density close to the Fermi level and exhibited a metallic state, demonstrating an increased conductivity in the composite. The adsorption models of sodium ions on Co₃S₄, Fe₇S₈, and Co₃S₄/Fe₇S₈ can be seen in Figure S6. According to Figure 7c, the calculated adsorption energy of Na on the Co₃S₄/Fe₇S₈ heterostructure (−2.33 eV) was much lower than that on Co₃S₄ (−0.23 eV) or Fe₇S₈ (−2.06 eV), demonstrating a more stable adsorption in the Co₃S₄/Fe₇S₈ heterostructure. The strong adsorption capability is due to the rich heterointerfaces that can supply more Na⁺ adsorption sites. To further reveal the electron flow direction at the heterointerface, the work functions of Co₃S₄ (5.60 eV) and Fe₇S₈ (5.47 eV) were calculated, as shown in Figure 7d,e. Through further calculation, the corresponding Fermi levels of Co₃S₄ (−0.41eV) and Fe₇S₈ (−2.04 eV) can be obtained. Based on this, electrons will transfer from the high Fermi level (Fe₇S₈) to the low Fermi level (Co₃S₄), thereby achieving a balance in the Fermi level and generating an intrinsic electric field, as shown in Figure 7f. In summary, benefitting from the positive effect of heterostructures, the electronic and ionic conductivities of Fe_0.88S/FeSe are simultaneously improved.

4. Conclusions

In summary, the unique Co₃S₄/Fe₇S₈ heterostructure was successfully prepared using the in situ self-crosslinking reaction mechanism coupled with a one-step sulfidation strategy. During the synthesis process, the difference in chelating ability between the abundant hydroxyl and carboxyl groups in sodium alginate with metal ions is fully utilized, and the controllable variation in the contents of Co₃S₄ and Fe₇S₈ in the material is achieved through the precise regulation of the Co/Fe molar ratio. Structural characterization and electrochemical measurements revealed that the as-fabricated SA-CoFe(1:4)-S electrode (Co/Fe = 1:4) exhibited a high initial discharge capacity of 1010 mAh·g⁻¹ at 1 A·g⁻¹, along with an outstanding cycling stability (a residual capacity of 258 mAh·g⁻¹ after 500 cycles at 3 A·g⁻¹) and excellent rate performance. Such enhanced electrochemical properties are predominantly ascribed to the synergistic interaction of the two components during Na⁺ storage and the in situ-formed heterojunction structure, which not only supplies abundant active sites but also expedites electron transfer and ion migration. This work offers valuable insights into the rational design and preparation of high-rate bimetallic sulfide anodes for sodium-ion storage.