Electrochemically self-driven integration of FeSe/FeS heterostructures for enhanced sodium storage and rapid kinetics

Abstract

An in situ electrochemical method is proposed to integrate FeSe/FeS heterostructures into a 3D S-doped carbon framework, enhancing sodium storage capacity and kinetics. Concurrently, both in situ and ex situ techniques are employed to investigate the underlying mechanisms.

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Rechargeable sodium-ion batteries (SIBs) are recognized for their cost-effectiveness and safety, making them promising candidates for large-scale energy storage.^1,2 Hard carbon is the current leading anode material, but its low capacity and sluggish kinetics limit the energy and power densities of SIBs.³ Transition metal selenides (TMSs) offer an alternative, with high capacity and redox reversibility.⁴ However, their low conductivity and significant volume changes during cycling hinder practical applications. Nanostructuring and carbon modification are common strategies to enhance kinetics and stability.⁵ Nanosized TMSs improve Na⁺ diffusion and reduce internal stress, but face challenges like particle aggregation due to high surface energy.⁶

Electronic structure engineering is key to unlocking the full potential of TMS electrodes. Creating heterostructures by combining nanocrystals with different band gaps enhances performance by reducing ion-diffusion resistance and improving electron transfer.⁷ For example, Guo et al.⁸ demonstrated that SnS/SnO₂ heterostructures on graphene nanosheets excellent rate capability and long cycle life, while Na₂S/Na₂Se heterostructures improve diffusion kinetics due to minimal interfacial deformation and matched lattice constants.⁴ Despite progress, research often overlooks the desodiation process. Maximizing Na₂S/Na₂Se interfaces in metal S/Se hybrids is crucial for boosting anode performance, though the exact mechanisms behind their synergy remain unclear. Understanding the relationship between active sites and reaction kinetics is essential for optimizing these heterostructures.

Here, we present an in situ electrochemically self-driven strategy to design a precise FeSe/FeS heterostructure embedded in S-doped carbon (FeSe/FeS-SC). This heterostructure, formed via electrochemical reactions from FeSe_0.8S_0.2 during initial cycling, shortens Na⁺ diffusion paths, enhances electron transfer, and maintains structural integrity. Experimental and theoretical studies confirm that the in situ FeSe/FeS formation improves conductivity and accelerates Na⁺ diffusion. Solid-state NMR shows that Na₂S/Na₂Se interfaces provide additional sodium adsorption sites, boosting specific capacity. The electrode achieves a high capacity of 498 mA h g⁻¹ at 1 A g⁻¹, excellent rate capability of 421 mA h g⁻¹ at 15 A g⁻¹, and long-term stability, retaining 356 mA h g⁻¹ after 3000 cycles at 3 A g⁻¹.

The synthesis of FeSe/FeS-SC is illustrated in Fig. 1a. The morphology and microstructure of the as-prepared FeSe_0.8S_0.2-SC were investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Fig. 1b and c demonstrate that FeSe_0.8S_0.2-SC exhibits a three-dimensional porous architecture characterized by uniformly distributed nanospheres (∼30 nm) embedded within a carbon scaffold. High-resolution TEM (HRTEM) (Fig. 1d and e) shows lattice fringes of 0.165 nm corresponding to the (103) plane of FeSe. Lattice distortions and vacancies (Fig. 1f and g) confirm high defect levels, enhancing conductivity and providing active sites for sodium storage. X-ray spectroscopy (EDX) mapping (Fig. 1h) shows uniform distribution of Fe, Se, S, and C elements. X-ray diffraction (XRD) (Fig. 1i) reveals peaks at 32°, 42°, and 50°, confirming the structure preservation with partial S substitution. Raman spectra (Fig. 1j) show increased I_D/I_G ratio for FeSe_0.8S0₂-SC, indicating higher disorder due to S incorporation.⁹ To determine the carbon content in FeSe_0.8S_0.2-SC, thermogravimetric analysis was conducted from 30 to 800 °C under air atmosphere (Fig. S1, ESI†). The carbon content was calculated to be 26 wt%. Electron paramagnetic resonance (EPR) (Fig. S2, ESI†) detects Se vacancies with a g value of 2.002.10 X-ray photoelectron spectroscopy (XPS) analysis (Fig. S3, ESI†) confirmed Fe, Se, S, and C in FeSe_0.8S_0.2-SC. The high-resolution Fe 2p spectrum (Fig. 1k) presents peaks at 724.6 eV and 711.1 eV corresponding to Fe²⁺ 2p_1/2 and Fe²⁺ 2p_3/2, respectively,¹⁰ as well as 726.3 eV and 713.3 eV for Fe³⁺ 2p_1/2 and Fe³⁺ 2p_3/2, along with satellite peaks at 731.5 eV and 718.9 eV. A shift to lower binding energy compared to FeSe-C suggests S incorporation.¹¹ Se 3d spectra (Fig. 1l) show peaks at 55.5 eV, 56.4 eV, 58.6 eV, and 59.5 eV (Se 3d_5/2, Se 3d_3/2, Se–C, and Se–O),¹² with shifts indicating S incorporation into the lattice.¹³ S 2p spectra (Fig. S4, ESI†) show peaks at 165.2 eV, 164.0 eV, 167.7 eV, and 161.8 eV (S 2p_1/2, S 2p_3/2, Se 3p_1/2, and Se 3p_3/2),¹⁴ confirming S doping.¹⁵ The C 1s spectrum (Fig. 1m) show peaks at 284.8 eV, 286.1 eV and 289.3 eV (C–C, C–O/C–S, and C=O),¹⁶ with C–S bond with C–S bonding further indicating S incorporation into the carbon matrix.¹⁷

Fig. 1 (a) Schematic illustration of the formation process of FeSe_0.8S_0.2-SC composite. (b) SEM image of FeSe_0.8S_0.2-SC. (c)–(e) TEM and HRTEM images of FeSe_0.8S_0.2-SC. (f) Lattice distortion. (g) Vacancy sites. (h) Elemental mapping signals. (i) XRD pattern. (j) Raman spectra and XPS spectra of (k) Fe 2p, (l) Se 3d, and (m) C 1s in FeSe_0.8S_0.2-SC.

To investigate the in situ electrochemical transformation from FeSe_0.8S_0.2-SC to FeSe/FeS-SC, both in situ and ex situ measurements were performed during sodiation/desodiation. In situ XRD analysis at 0.2 A g⁻¹ recorded a series of patterns (Fig. 2a). Initially, the peaks at 32.5° and 42.5° correspond to the (004) and (114) planes of FeSe_0.8S_0.2. During discharge (open circuit to 1 V), these peaks shift to lower angles, indicating Na⁺ insertion and lattice expansion.¹⁸ By 0.1 V, these peaks disappear, and new peaks at 37.0° and 22.5° emerge, corresponding to Na₂S and Na₂Se,¹⁹ indicating a conversion reaction with FeSe_0.8S_0.2. No Fe peaks are detected, likely due to the nanograin size and poor crystallinity of Fe. During desodiation, the peaks of Na₂S and Na₂Se weaken, and by 3 V, these vanish entirely. The absence of FeSe_0.8S_0.2 and the appearance of FeSe and FeS suggest phase transformation through an electrochemical-driven process. Ex situ TEM (Fig. 2b) at 1 V reveals lattice fringes corresponding to Fe(110), Na₂Se(200), and Na₂S(201), confirming the conversion to Fe, Na₂Se and Na₂S. The persistence of FeSe_0.8S_0.2 indicates an incomplete reaction. At 0.01 V (Fig. 2c), only Fe, Na₂Se, and Na₂S are present, completing the conversion reaction. The Na₂Se and Na₂S form distinct interfaces within the same particles. Upon charging to 3 V (Fig. 2d), Na₂Se and Na₂S vanish, and FeSe and FeS are detected, confirming the formation of FeSe/FeS-SC through an electrochemical self-driven process. The in situ electrochemical self-driven process can be summarized as Fig. 2e.

Fig. 2 (a) In situ XRD patterns of FeSe_0.8S_0.2-SC and corresponding GCD curves during initial cycle at 200 mA g⁻¹. HRTEM images of FeSe_0.8S_0.2-SC electrode at different initial discharged and charged states: (b) discharged state of 1.0 V; (c) discharged state of 0.01 V; (d) charged state of 3 V. (e) Schematic illustration of the electrochemical reaction for FeSe_0.8S_0.2-SC electrode.

Sodiation:

FeSe_0.8S_0.2 + xNa⁺ + xe⁻ → Na_xFeSe_0.8S_0.2 (1)

Na_xFeSe_0.8S_0.2 + (4 − x)Na⁺ + (4 − x)2e⁻ → Na₂Se + Na₂S + Fe (2)

Desodiation:

Na₂Se + Na₂S + 2Fe → FeSe + FeS + 4Na⁺ + 4e⁻ (3)

The sodium ion storage performance of the synthesized FeSe/FeS-SC sample was evaluated using a coin cell with Na metal as the counter electrode. Fig. 3a shows the cyclic voltammetry (CV) profiles for the first four cycles at 0.1 mV s⁻¹. The peak at ∼1.1 V corresponds to the conversion of FeSe_0.8S_0.2-SC to Fe and the formation of the SEI layer, followed by the reverse transformation to FeS and FeSe. Faint redox peaks around 0.15/0.1 V suggest Na⁺ insertion/extraction in carbon. Stable anodic peaks at 1.9, 1.3, and 0.7 V in subsequent cycles indicate Na⁺ insertion into FeSe and FeS, forming Na₂Se, Na₂S, and Fe. The well-overlapped CV curves from the 2nd to 4th cycles demonstrate the excellent reversibility of the FeSe/FeS-SC electrode. Fig. 3b displays the charge–discharge profiles, with the electrode achieving an initial discharge/charge capacity of 569 and 516 mA h g⁻¹, and a high initial coulombic efficiency (ICE) of 90.8%. which stabilizes at ∼100% after the second cycle. The superior rate capability (Fig. 3c) shows that the electrode delivers 518 mA h g⁻¹ at 0.2 A g⁻¹ and maintains 421 mA h g⁻¹ at 15 A g⁻¹. Upon returning to 0.2 A g⁻¹, the capacity is fully recovered, while the FeSe-C electrode exhibits lower capacities at all current densities, only reaching 195 mA h g⁻¹ at 15 A g⁻¹. Long-term cycling stability at 3 A g⁻¹ (Fig. 3d) demonstrates a capacity retention of 82.4% over 3000 cycles. To demonstrate its excellent structural stability, TEM characterization was conducted after 100 cycles (Fig. S5, ESI†). The results show that the three-dimensional porous structure is well maintained, thereby ensuring its superior cycling stability. Compared to currently reported metal sulfide anodes, the FeSe/FeS-SC anode exhibits superior rate capability and cycling stability (Table S1, ESI†). GITT analysis (Fig. S6 and S7, ESI†) reveals higher Na⁺ diffusion coefficients (D_Na⁺) for the FeSe/FeS-SC electrode compared to FeSe-C (Fig. 3e), indicating improved Na⁺ transport due to the FeSe/FeS heterostructure within the carbon matrix. Fig. 3f shows that CV profiles at various scan rates maintain similar shapes with minimal peak shifts, signifying superior reversibility and low polarization. The b values derived from the equation i = av^b (ref. 20) (Fig. 3g) consistently exceed 0.8, implying capacitive-controlled Na⁺ storage. Further analysis using i = k₁v + k₂v^1/2 (ref. 21) (Fig. 3h) shows that at 0.2 mV s⁻¹, pseudocapacitance contributes 87.3% of total charge storage, increasing to 97.3% at 3 mV s⁻¹ (Fig. S8, ESI†). This increase in capacitive contribution is attributed to the multi-interfaces in the FeSe/FeS-SC electrode, which enhance electrochemical kinetics and rate performance. A full cell was assembled using sodium vanadium phosphate (NVP) as the cathode and FeSe/FeS-SC as the anode, with tests conducted over a voltage range of 1–3.7 V. Fig. S9a (ESI†) presents the initial charge–discharge curves, showing a coulombic efficiency of 79%, indicative of efficient ion transport and favorable electrode compatibility. Fig. S9b (ESI†) illustrates the cycling stability of the cell over 150 cycles, where the capacity remains stable at 380 mA h g⁻¹, demonstrating excellent long-term performance and retention.

Fig. 3 (a) CV curves of FeSe/FeS-SC at scanning rate of 0.1 mV s⁻¹. (b) Discharge/charge curves of FeSe/FeS-SC at 0.2 A g⁻¹. (c) Rate performance of FeSe/FeS-SC and FeSe-C. (d) Cycling performance of FeSe/FeS-SC at 3 A g⁻¹, inserted figure shows the first 20 cycles. (e) Na⁺ diffusion coefficients of FeSe/FeS-SC and FeSe-C. (f) CV curves in FeSe/FeS-SC at different scanning rate from 0.1 to 3 mV s⁻¹. (g) b values at different voltages during the cathodic scan. (h) Capacitive contribution of FeSe/FeS-SC at a scan rate of 0.2 mV s⁻¹.

The high capacity of the FeSe/FeS-SC electrode was analyzed using ex situ solid-state nuclear magnetic resonance (NMR). Fig. 4a shows ²³Na NMR spectra at various states. At 1 V discharge, four peaks appear: 24.9 ppm for the solid electrolyte interphase (SEI), 5.1 ppm for sodium ions in the FeSe_0.8S_0.2-SC matrix, and −7.5 and −22 ppm for Na⁺ at the heterointerface and adsorbed on Fe,²² respectively. These findings align with ex situ TEM results indicating Fe particle formation and the creation of active Na₂S/Na₂Se heterointerfaces. Discharging to 0.01 V enhances the peak at 4.7 ppm, indicating increased sodium ion concentration, crucial for capacity. Upon recharging to 1 V, peaks related to Na⁺ at the heterointerface diminish, signifying Na⁺ extraction. By 3 V, only SEI and embedded sodium ion peaks remain, reflecting reduced free sodium ions after the reverse reaction. Fig. 4b outlines the electrochemical process: Na₂S/Na₂Se interfaces and embedded sodium ions serve as active sites for Na⁺. Metallic iron also adsorbs sodium due to the space charge effect. Some sodium becomes irreversible during charging, contributing to initial capacity loss, while the heterointerface's gradual reduction underscores the reaction's high reversibility.

Fig. 4 (a) Experimental and simulated ²³Na NMR spectra of FeSe/FeS-SC electrodes at different states of charge. (b) Schematic diagram of sodium storage mechanism of FeSe/FeS-SC.

To explore interfacial interactions in the FeSe/FeS-SC heterostructure, density functional theory (DFT) calculations were performed. Figure S10a and b (ESI†) show that both FeSe-C and FeSe/FeS-SC exhibit conductive properties with high electron density at the Fermi level, while the introduction of FeS enhances electrical conductivity. The binding energy (E_b) for Na ions, calculated as E_b = E_host+Na − E_host − E_Na, indicates favorable binding, with FeSe/FeS-SC showing an E_b of −1.92 eV, better that FeSe-C's −1.52 eV (Fig. S10c, ESI†). Lower energy barriers for Na diffusion in FeSe/FeS-SC (Fig. S10d, ESI†) facilitate rapid kinetics, improving rate capability. Bader charge calculations (Fig. S10e, ESI†) reveal that Fe atoms lose electrons, increasing negativity at Se and S sites for better Na⁺ adsorption. The electron localization function (ELF) (Fig. S10f, ESI†) shows reduced electron delocalization around Fe, enhancing conversion reactivity. In summary, DFT calculations demonstrate that the FeSe/FeS heterostructure effectively traps Na⁺ ions and improves ionic conduction, enhancing overall performance and reaction reversibility.

In summary, we developed an innovative in situ electrochemically self-driven strategy to integrate FeSe/FeS heterostructures within a 3D S-doped carbon framework. In situ XRD and ex situ TEM analyses revealed the irreversible conversion of FeSe_0.8S_0.2 to FeSe and FeS during the first charge cycle. Experimental and density functional theory (DFT) analyses indicated that fast diffusion kinetics at the FeSe/FeS heterointerface result from small interface deformation, similar electronegativity of sulfur and selenium, and comparable lattice constants. These factors facilitate additional charge transfer, enhancing electrochemical kinetics. The active sites within the heterointerface significantly improve Na⁺ ion adsorption, leading to superior charge storage. The engineered electrode achieved a capacity of 498 mA h g⁻¹ at 1 A g⁻¹, maintained a remarkable rate capability of 421 mA h g⁻¹ even at 15 A g⁻¹, and demonstrated robust cycling stability with a retention of 356 mA h g⁻¹ after 3000 cycles at a 3 A g⁻¹. These findings highlight the potential of our approach for developing high-performance sodium-ion batteries (SIBs) and provide valuable insights for future research and applications in the field.

Hui Li: writing – original draft, validation, investigation, data curation. Guangshuai Han: writing – review & editing. Yanchen Fan: writing – review & editing. Xin Zhang: data curation. Haoxi Ben: resources, funding acquisition. Chunrong Ma and Zhaoying Li: writing – review & editing, conceptualization, funding acquisition. Claire (Hui) Xiong: writing – review & editing.

This work was supported by State Key Laboratory of Bio-Fibers and Eco-Textiles, Qingdao University (No. G2RC202021) and Shandong Provincial Natural Science Foundation (ZR2021QE040).

Data availability

Data are contained within the article and ESI.†

Conflicts of interest

The authors affirm that they do not have any competing financial interests or personal relationships that could have influenced the work reported in this paper.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4cc05233a

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