Recent advances of conversion-type iron-based materials for sodium-ion batteries

Iron-based oxides

In recent years, iron-based oxides have gradually become the most promising electrodes due to their ultrahigh theoretical capacities and abundant natural availability^[11,17-21]. These oxides include FeO, Fe₃O₄, and Fe₂O₃, with Fe₂O₃ (theoretical specific capacity ~1,008 mA h g^-1) and Fe₃O₄ (926 mAh g^-1) being the most extensively studied^[11,17]. The sodium-ion storage mechanism of Fe₂O₃ in SIBs is as follows^[21-24]:

However, iron oxide electrodes suffer from several inherent shortcomings: (1) the generation of electrochemically inactive Na₂O during cycling severely degrades cycle stability; (2) the intrinsic low conductivity leads to sluggish ion diffusion, poor rate capability, and significant voltage hysteresis; (3) massive volume expansion and Fe⁰ nanoparticle agglomeration during cycling cause structural damage and rapid capacity decay^[17,22-24].

To overcome these challenges, researchers have designed various nanostructures and incorporated conductive coatings on iron-based materials to enhance electronic conductivity and structural integrity^[15,17]. For instance, Li et al. designed a hierarchical Fe₂O₃@MIL-101(Fe)/C anode via a MOF-derived method^[18]. The hierarchical nanostructure increases the surface areas, facilitating electrolyte infiltration and shortening diffusion lengths for both Na⁺ ions and electrons^[18]. Furthermore, the intrinsic hollow architecture helps mitigate volume changes during Na⁺ insertion/extraction. As a result, Fe₂O₃@MIL-101(Fe)/C exhibits excellent cycling stability, delivering a capacity of 662 mAh g^-1 over 200 cycles at 200 mA g^-1 with 93.2% capacity retention^[18].

Graphene, known for its ultrahigh surface area and exceptional electronic conductivity, is also a highly promising component in electrode design. Li et al. synthesized Fe₂O₃ nanoparticles embedded in N-doped graphene with internal micro-channels (Fe₂O₃@N-GIMC)^[19]. The interconnected porous structure offers abundant active sites and efficient ion pathways, while also mitigating nanoparticle aggregation and pulverization. Consequently, Fe₂O₃@N-GIMC exhibits outstanding Na⁺ storage performance, achieving a capacity of 308.9 mAh g^-1 over 1,000 cycles and 200.8 mAh g^-1 over 4,000 cycles at 1 A g^-1[19].

To alleviate the volumetric strain and enhance the electronic conductivity of Fe₂O₃ anodes, Chen et al. encapsulated a Fe₂O₃ core in N-doped carbon nanosphere shells (MFe₂O₃@N-HCNs) through a simple confined impregnation crystallization method [Figure 2A]^[20]. The as-obtained MFe₂O₃@N-HCNs not only possess a connected hierarchical structure that shortens the diffusion length, but also effectively accommodate the significant volumetric stress during Na⁺ insertion/extraction [Figure 2B and C]. Figure 2D shows a lattice spacing of 0.20 nm, which corresponds to the (410) crystal plane of Fe₂O₃. As a result, the MFe₂O₃@N-HCNs exhibit outstanding rate capability, achieving specific capacities of 454, 360, 309, 257, and 201 mAh g^-1 at current densities of 0.1 to 2 A g^-1, respectively [Figure 2E]. Additionally, Figure 2F demonstrates that the MFe₂O₃@N-HCNs anode maintains superior cycling stability, retaining a capacity of 417 mAh g^-1 after 100 cycles at 0.1 A g^-1[20].

Figure 2. (A) Schematic diagram of the preparation process of MFe₂O₃@N-HCNs; (B) SEM, (C) TEM, and (D) HRTEM images of MFe₂O₃@N-HCNs. (E) Rate capabilities of N-HCNs, MFe₂O₃, and MFe₂O₃@N-HCNs anodes from 0.1 to 2 A g^-1; (F) Cycling performance of N-HCNs, MFe₂O₃, and MFe₂O₃@N-HCNs at 0.1 A g^-1[20]. Copyright 2021, American Chemical Society.

Reduced graphene oxide (rGO) offers significant electronic conductivity and a high surface area, making it a promising composite material. Li et al. synthesized a composite of amorphous Fe₂O₃/graphene nanosheets (Fe₂O₃@GNS) via a simple procedure^[21]. They found that amorphous ultrafine Fe₂O₃ particles (~5 nm) were uniformly riveted onto the GNS via strong oxygen-bridge (C-O-Fe) bonds. This composite anode exhibited superior Na⁺ storage performance compared to well-crystallized Fe₂O₃. Specifically, theFe₂O₃@GNS anode delivered a capacity of 440 mAh g^-1 at 0.1 A g^-1, yielding an initial Coulombic efficiency (ICE) of 81.2%. Even at a high current density of 2 A g^-1, it retained a capacity of 219 mAh g^-1. The strong interactions between GNS and Fe₂O₃ not only provided abundant active sites and facilitated Na⁺/electron diffusion but also mitigated volume changes during discharging/charging^[21]. Furthermore, Li et al. prepared single-crystal Fe₂O₃ particles (~300 nm) uniformly distributed on rGO nanosheets (Fe₂O₃/rGO)^[22]. When used as an anode for SIBs, this Fe₂O₃/rGO composite demonstrated an ICE of 71% and retained a capacity of 610 mAh g^-1 at 0.05 A g^-1, with 82% retention after 100 cycles^[22]. Bhar et al. developed a free-standing anode by embedding Fe₂O₃ nanoparticles within a carbon fiber (CF) network, effectively mitigating the conductivity and pulverization issues associated with Fe₂O₃^[23]. Their Fe₂O₃-CF anode delivered a discharge capacity of 1,154 mAh g^-1, substantially higher than that of conventional Fe₂O₃ anodes (653 mAh g^-1), and demonstrated a capacity retention of 70% over 100 cycles^[23]. Zhao et al. constructed Fe₂O₃ nanorods directly on carbon cloth via a simple hydrothermal strategy to develop a free-standing SIB anode^[24]. These Fe₂O₃ nanorods achieved an impressive specific capacity retention of 119% after 100 cycles^[24]. In another study, Shi et al. in situ synthesized amorphous Fe₂O₃ on N-doped carbon (NC) nanofibers to form a novel Fe₂O₃/NC anode^[25]. This anode achieved a Na⁺ storage capacity of 408 mAh g^-1 over 350 cycles at 0.1 A g^-1 and maintained 183 mAh g^-1 at 3 A g^-1[25]. The outstanding electrochemical features of these free-standing anodes are related to the synergistic effects between the nanostructured Fe₂O₃ and the cross-connected carbon fiber network, which not only alleviates volumetric strain and provides more active storage sites, but also enhances the overall electrode conductivity, thereby accelerating Na⁺/electron diffusion during cycling.

Except for Fe₂O₃, Fe₃O₄ possesses a high theoretical capacity of 926 mAh g^-1, along with advantages such as safety, non-toxicity, environmental friendliness, abundant availability, and low cost. These properties have attracted significant attention to Fe₃O₄ as a promising anode material for SIBs^[26-30]. The Na⁺ storage mechanism of Fe₃O₄ proceeds as follows^[26-30]:

However, similar to other transition metal oxides, Fe₃O₄ suffers from substantial volume expansion during cycling. This volume change causes structural degradation and results in poor cycle stability, thereby limiting its practical application^[27-31]. To address this challenge, Liu et al. synthesized a composite of graphene and Fe₃O₄ nanodots through a simple hydrothermal procedure and applied it as an anode material for SIBs^[26]. This unique hierarchical structure provides a large solid electrolyte interphase (SEI) that facilitates rapid Na⁺/electron diffusion, mitigates volumetric strain, and prevents aggregation of nano-Fe₃O₄ during discharging/charging. Consequently, the graphene-Fe₃O₄ quantum dot composite achieved a high specific capacity of 525 mAh g^-1 at 0.03 A g^-1 and exhibited excellent cycling stability, retaining 312 mAh g^-1 after 200 cycles at 50 mA g^-1[26]. To further overcome the low conductivity of Fe₃O₄, Tao et al. developed an N/S/P-doped dual carbon-modified Fe₃O₄ nanoparticle (Fe₃O₄@C@G)^[27]. As illustrated in Figure 3A, the N/S/P-doped carbon shell was formed by pyrolyzing a multi-heteroatom-containing polymer, resulting in a core-shell Fe₃O₄@C structure, which was subsequently encapsulated with rGO to form Fe₃O₄@C@G^[27]. SEM images [Figure 3B] show that numerous nanospheres are uniformly distributed on the rGO layers. TEM images [Figure 3C and D] further confirm that Fe₃O₄ nanoparticles with carbon shells are wrapped by rGO. The initial six CV curves of Fe₃O₄@C@G [Figure 3E] show broad cathodic/anodic peaks within the ranges of 1.0~1.25 and 1.25~1.5 V, respectively, corresponding to the reversible redox reactions described in Eq. 2. From the second cycle onward, the CV curves largely overlap, suggesting excellent electrochemical reversibility of the Fe₃O₄@C@G anode. When applied as an anode for SIBs, Fe₃O₄@C@G delivered a capacity of 180 mAh g^-1 over 600 cycles at 0.1 A g^-1 [Figure 3F]^[27]. Biswal et al. synthesized a dual core@shell Fe₃O₄@C@polypyrrole (PPy) composite anode^[28]. This material showed a capacity of 64 mAh g^-1 over 100 cycles at 0.1 A g^-1[28].

Figure 3. (A) Schematic illustration of the preparation of Fe₃O₄@C@G. (B) SEM, (C) TEM, and (D) HRTEM images of Fe₃O₄@C@G. (E) CV curves of Fe₃O₄@C@G; (F) Comparison of the cycling performance of Fe₃O₄, Fe₃O₄@C, and Fe₃O₄@C@G at 0.1 A g^-1[27]. Copyright 2020, Elsevier.

Conventional carbon coating strategies primarily enhance electrical conductivity and mitigate volume expansion at the macro scale. In contrast, metal-organic framework (MOF)-derived approaches have emerged as effective nanoscale engineering methods, enabling the in situ encapsulation of metal oxide quantum dots during pyrolysis^[29]. Qi et al. developed Fe₃O₄ quantum dots embedded in a raspberry-like carbon framework (Fe₃O₄ QD@C-GN) using iron-based MIL-88-Fe-NH₂ as a template^[29]. When tested within the voltage range of 0.05~3.0 V, Fe₃O₄ QD@C-GN exhibited capacities of 343, 234, and 149 mAh g^-1 at 2, 5 and 10 A g^-1, respectively, over 1,000 cycles^[29].

To address the issues of poor electronic conductivity and structural degradation induced by the high volume stress during cycling, Qin et al. creatively introduced a magnetic stimulus to increase pressure within the sample chamber. This approach facilitated the uniform deposition of red P particles onto a chain-like Fe₃O₄/C composite, forming Fe₃O₄/C/P. When used as an anode material and cycled within a voltage range of 0.01~3 V, the Fe₃O₄/C/P structure exhibited excellent cycling stability and outstanding rate capability^[30]. This magnetic field-assisted strategy effectively bridges the synthesis method with enhancements in electrochemical properties. Huang et al. encapsulated Fe₃O₄ in N-doped carbon nanofibers (CFs) through an electrospinning strategy followed by high-temperature calcination. The resulting three-dimensional (3D) interconnected CF network enhanced overall electronic conductivity, while N-doping introduced additional sites for Na⁺ storage. Hence, the optimized Fe₃O₄-CFs achieved a considerable capacity of 358.1 mAh g^-1 over 200 cycles at 0.5 A g^-1[31].

Although structural regulation, composite modification, and surface engineering have markedly improved the electrochemical performance of iron-based oxides for SIBs, further enhancements are still required to meet the demands of practical applications.

Iron-based sulfides

Compared to Na₂O, Na₂S exhibits higher electrochemical reactivity, and iron-based sulfide materials have also attracted considerable attention^[20,31-34]. These materials offer relatively high theoretical capacities and are composed of naturally abundant, non-toxic and cost-effective elements (e.g., Fe and S)^[35-39]. To date, a series of iron-based sulfides have been explored. Among them, FeS₂ has received particular attention due to its ultrahigh capacity of 894 mAh g^-1 resulting from a four-electron/Na⁺ transfer per FeS₂ unit. Kitajou et al. investigated the discharge/charge mechanism of FeS₂ using synchrotron-based X-ray adsorption near-edge structure (XANES), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD)^[32]. The Na⁺ storage process in FeS₂ proceeds as follows^[32-35]:

The main contributors to the irreversible capacity are (i) the dissolution of S₂^2- ions during charging/discharging; and (ii) the inevitable formation of a SEI layer, both of which result in low ICE^[36-40]. To address these challenges, strategies such as nanochemistry, structural regulation, and surface modification have proven effective^[39-42]. For instance, Li et al. demonstrated through ex situ TEM measurements that only a portion of FeS₂ actively participates in the conversion reaction, while the formation of “inactive cores” leads to low capacity^[33]. Moreover, the sluggish diffusion kinetics induced by the relatively larger Na⁺ ionic radius exacerbate electrode pulverization, thereby causing rapid capacity decay^[33]. Zhang et al. developed mesoporous FeS₂ nanorods via a scalable synthesis method^[34]. Due to their unique one-dimensional porous structure and excellent strain relaxation capabilities, these nanorod anodes exhibited outstanding Na⁺ storage performance, maintaining a reversible capacity of 711.1 mAh g^-1 over 450 cycles at 1 A g^-1[34].

Despite the high theoretical capacity of FeS₂, its practical application remains limited due to severe volume fluctuations during charging/discharging and the formation of soluble polysulfides at low voltages (< 0.8 V vs. Na/Na⁺). Kandula et al. designed N-doped carbon-coated FeS₂ nanoparticles wrapped in N/S co-doped graphene/single-walled carbon nanotubes (FSCGS) via hydrothermal sulfuration^[35]. When used as an anode for SIBs, the FSCGS composite achieved a superior rate capability (476 mAh g^-1 at 10.0 A g^-1). Furthermore, it demonstrated excellent stability with a capacity retention of 91.3% over 200 cycles at 0.5 A g^-1[35]. The superior performance is attributed to the enhanced electron conductivity and electrochemical reactivity provided by the dual-carbon-modified hierarchical structure. Ma et al. designed a core-shell hollow sphere FeS₂ (CHS-FeS₂) using MIL-88B(Fe) as a template^[36]. Its unique porous structure provides ample void space to buffer volume strain and promote Na⁺ diffusion. As a result, the CHS-FeS₂ anode maintained a capacity of 546.5 mAh g^-1 over 100 cycles at 0.5 A g^-1 and retained 224 mAh g^-1 even at 20 A g^-1, reflecting its remarkable Na⁺ storage performance^[36]. Lu et al. embedded ultrafine FeS₂ particles into N/S co-doped CFs, along with FeS₂ nanoflakes, forming a composite (FeS₂@CF-NS) via electrospinning followed by annealing^[37]. This unique architecture effectively reduces the Na⁺ diffusion path, accelerates Na⁺/e^- diffusion, and mitigates volume strain during discharging/charging. The FeS₂@CF-NS composite demonstrated a high specific capacity of 637.1 mAh g^-1 after 400 cycles at 1 A g^-1 and retained 431.1 mAh g^-1 at 5 A g^-1[37]. Ma et al. synthesized a series of Co_xFe_1-xS₂ compounds with varying Co content via a typical solvothermal sulfuration procedure^[36]. Among them, the optimized Co_0.5Fe_0.5S₂ delivered a capacity of 220 mAh g^-1 over 5,000 cycles at 2 A g^-1, and 172 mAh g^-1 even at 20 A g^-1. Structural analysis revealed the formation of layered Na_xCo_0.5Fe_0.5S₂ during the initial discharging, which was preserved in subsequent cycles^[38]. Man et al. fabricated a yolk-shell FeS₂@C anode for SIBs^[39]. The porous yolk-shell design not only facilitates rapid Na⁺/e^- diffusion but also buffers the large volume changes of FeS₂ during Na⁺ insertion/extraction. The as-prepared FeS₂@C achieved a reversible capacity of 616 mAh g^-1 over 100 cycles at 0.1 A g^-1[39]. Liu et al. developed novel yolk-shell FeS₂@C nanoboxes using a facile etching procedure combined with a unique sulfidation-in-nanobox strategy^[40]. These nanoboxes displayed remarkable electrochemical properties, achieving a specific capacity of 330 mAh g^-1 over 800 cycles at 2 A g^-1[40].

Furthermore, Xu’s group successfully prepared pitaya-like FeS₂@C via a one-step in-situ encapsulation and hydrothermal sulfuration strategy, with ferrocene serving as both the iron and carbon source [Figure 4A]^[41]. FeS₂ particles are uniformly embedded in the porous carbon matrix, effectively preventing the agglomeration of Fe⁰ nanoparticles and minimizing the dissolution of S₂^2-. Furthermore, the porous structure facilitates Na⁺ ion diffusion, significantly improving the electrochemical performance of FeS₂. As depicted in Figure 4B, the pitaya-structured FeS₂@C exhibits an initial discharge/charge capacity of ~701/579 mAh g^-1 at 0.1 A g^-1. It maintains a capacity of 415 mAh g^-1 over 100 cycles at 0.6 A g^-1 [Figure 4C]. Figure 4D and E illustrates that FeS₂@C delivers stable reversible Na⁺ storage capabilities of 580, 502, 463, 422, 374, and 307 mAh g^-1 at current densities ranging from 0.1 to 3.2 A g^-1, respectively^[41]. Such excellent Na⁺ storage performance benefits from the porous carbon sphere framework, which effectively suppresses the shuttle effect of S_n^2- ions and contributes to excellent cycling stability. Simultaneously, the porous carbon matrix enhances the overall conductivity of the material. Furthermore, it acts as a flexible buffer, accommodating volume changes during Na⁺ insertion/extraction and thereby preserving the structural integrity of the electrode.

Figure 4. (A) Schematic illustration of the preparation process for pitaya-structured FeS₂@C. (B-E) electrochemical performance of FeS₂@C: (B) initial three charge/discharge profiles, (C) cycling performance at 0.6 A g^-1, (D) charge/discharge profiles from 0.1, 0.2, 0.4, 0.8, 1.6 to 3.2 A g^-1, respectively, (E) rate performance^[41]. Copyright 2017, American Chemical Society.

Chen et al. encapsulated graphene-coated FeS₂ in carbon nanofibers using an electrospinning strategy and employed it as an anode for SIBs^[42]. The double carbon modification improved electrical conductivity, thereby improving the structural reversibility of FeS₂ during desodiation/sodiation; thus, the material maintained a high capacity of 305.5 mAh g^-1 over 2,450 cycles at 3 A g^-1. Owing to the markedly enhanced conductivity of the graphene-encapsulated FeS₂ nanoparticles, the composite also exhibited excellent temperature tolerance, retaining stable capacity performance at 0 °C and -20 °C. When paired with Na₃V₂(PO₄)₃ in full-cell configurations, the system delivered reversible capacities of 95, 50, and 43 mAh g^-1 at room temperature, 0 °C, and -20 °C, respectively. Density functional theory calculations further revealed that dual carbon modification effectively reduces the Na⁺ diffusion barrier and facilitates reversible Na⁺ storage^[42].

FeS also possesses a high theoretical specific capacity (~609 mAh g^-1) and has been widely explored as an anode material for SIBs^[43,44]. Its Na⁺ storage mechanism involves a two-step conversion reaction, as described below^[45-49]:

FeS also suffers from unsatisfactory electrical conductivity and significant volumetric strain leading to rapid capacity decay and poor rate performance^[43,49]. To address these issues, Cho et al. fabricated yolk-shell structured FeS/C composites via a vapor-pressure-induced synthesis route^[43]. The ultrathin carbon coatings on the FeS nanosheets effectively improved their electrical conductivity and mitigated the volumetric strain during sodiation. As a result, the FeS/C yolk-shell nanosheets delivered a reversible capacity of 300.4 mAh g^-1 over 10,000 cycles, with 81.1% capacity retention at 10 A g^-1[43]. Han et al. prepared porous FeS nanofiber anodes for SIBs via electrospinning followed by sulfidation^[44]. During synthesis, ultrafine Fe₂O₃ nanocrystals diffused to the fiber surface through Ostwald ripening and then transferred into porous FeS sulfidation. The resulting FeS nanofibers featured abundant nanovoids that help buffer volumetric strain and maintain structural stability, thereby enabling excellent Na⁺ storage properties. These nanofibers exhibited an initial discharge capacity of 561 mAh g^-1 and maintained 592 mAh g^-1 at 0.5 A g^-1 over 150 cycles. When cycled at increasing current densities from 0.2 to 5.0 A g^-1, the FeS anode retained capacities of 456, 437, 413, 394, 380, and 353 mAh g^-1, respectively^[44]. Liu et al. synthesized a FeS/NC composite via an in situ chemical transformation method^[45]. In this design, FeS nanocrystals were encapsulated by the in-situ generated N-doped carbon (NC) matrix. The FeS/NC anode exhibited a high charge capacity of 511 mAh g^-1 over 100 cycles at 0.2 A g^-1, and retained 326 mAh g^-1 after 500 cycles at 1.0 A g^-1. These excellent electrochemical results are ascribed to the NC matrix, which enhances electrical conductivity and accommodates the volume changes of FeS during Na⁺ insertion/extraction^[45]. Chen et al. developed a Mn-doped FeS/NC composite through a one-pot solvothermal reaction followed by thermal treatment^[46]. Mn doping improved electrical conductivity, facilitated charge carrier transport, and expanded lattice spacing to promote Na⁺ diffusion. The Mn-doped FeS/NC anode achieved a specific capacity of 563.3 mAh g^-1 at 0.5 A g^-1, maintained 442.8 mAh g^-1 at 8 A g^-1, and retained 206.2 mAh g^-1 over 8,000 cycles^[46].

The practical application of FeS anodes is hindered by their poor electronic conductivity and significant volume changes during cycling. Yuan et al. synthesized ultrafine FeS nanoparticles encapsulated in three-dimensional (3D) NC nanosheets using a facile sol-gel approach followed by a solid-state sulfidation process [Figure 5A]^[47]. As displayed in Figure 5B, the sol-gel-derived precursor forms a 3D cross-linked carbon framework upon thermal treatment. After sulfidation, FeS nanoparticles are uniformly embedded in the carbon nanosheets [Figure 5C and D]. When used as an anode material for SIBs, the 3D FeS@NC composite delivered a stable capacity of 254 mAh g^-1 over 1,100 cycles at 1.5 A g^-1 [Figure 5E]^[47]. These remarkable electrochemical features are associated with the 3D porous nanosheet structure, which dramatically enhances electronic conductivity, suppresses the pulverization and aggregation of FeS particles, immobilizes dissolved polysulfides, increases the electrolyte-electrode contact area, and provides abundant active sites for Na⁺ storage.

Figure 5. (A) Schematic of the preparation of FeS@NC. (B) SEM image of Fe@NC; (C and D) SEM images of FeS@NC, (E) long-term cycling performance of FeS@NC at 1.0 A g^-1[47]. Copyright 2022, Elsevier.

The incompatibility between electrodes and electrolytes significantly limits the electrochemical performance of iron-based sulfides. Poor interfacial compatibility leads to unstable solid electrolyte interphase (SEI), resulting in persistent irreversible side reactions that consume active Na⁺ ions. On account of these, Zhang et al. synthesized FeS@N,S-C, comprising spindle-structured FeS nanoparticles wrapped in N,S-doped carbon, as an anode material for SIBs^[48]. They investigated the interfacial stability in an ester-based electrolyte (1 M NaClO₄ in propylene carbonate/ethylene carbonate with 5% fluoroethylene carbonate). The Na//FeS@N,S-C half-cell demonstrated a long cycling lifespan of 294 days. Furthermore, the corresponding pouch cell displayed excellent cyclability, retaining 82.2% of its capacity over 170 cycles at 0.2 A g^-1. Their findings suggest that the N,S-doped coating layer preferentially adsorbs ClO₄^- ions, inducing strong repulsion toward solvated Na⁺ ions and creating an anion-rich inner Helmholtz plane. This results in the formation of an inorganic species-rich SEI (mainly NaCl and Na₂O), which preserves the electrode’s structural integrity^[46].

Considering the challenges posed by dissolved polysulfides and significant volume expansion, Huang et al. synthesized a bubble film-like FeS/C composite using Fe(NO₃)₃·9H₂O, sulfur, and polyvinylpyrrolidone as precursors, followed by freeze-drying and carbonization^[49]. The bubble film-like FeS/C exhibited reversible Na⁺ storage capacities of 455, 439, 405, 318, 256, and 219 mA h g^-1 at current densities ranging from 0.1 to 5.0 A g^-1, respectively^[49]. The superior electrochemical performance is mainly attributed to the porous bubble film-like structure, which effectively suppresses the agglomeration of active nanoparticles, thereby enhancing cycling stability. Additionally, the porous carbon framework improves overall electrical conductivity, enabling excellent rate capability and high capacity retention. Moreover, the porous structure provides ample void space that mitigates volume expansion and maintains the structural integrity during charging/discharging.

Iron-based selenides

The SEI formed during electrochemical reactions can hinder the transport of Na⁺ ions, resulting in the low practical capacity of oxide-based anodes for SIBs^[48-61]. As for sulfides, the formation of soluble polysulfide intermediates induces a “shuttle effect”, which aggravates capacity decay^[50-54]. In contrast, metal selenides exhibit relatively high electronic conductivity and energy conversion efficiency, making them promising electrode materials for SIBs^[55-59]. Among the various selenides studied to date, FeSe₂ and Fe₇Se₈ are two of the most representative anode materials^[58-61]. The Na-ion storage mechanism of FeSe₂ is as follows^[58-63]:

However, the sluggish diffusion kinetics and severe volume expansion during cycling lead to rapid capacity fading, hampering the practical application of FeSe₂ in SIBs. Wei et al. synthesized nanorod-assembled FeSe₂ clusters via a hydrothermal strategy and investigated their Na⁺ storage performance under various electrolyte conditions^[50]. When paired with an ether-based electrolyte, the FeSe₂ clusters demonstrated superior electrochemical properties, delivering a high capacity of 515 mAh g^-1 over 400 cycles at 1 A g^-1 and a high ICE of 97.4%. Remarkably, they retained 128 mAh g^-1 even at an ultrahigh current density of 35 A g^-1[50]. Additionally, Wang et al. prepared NC-encapsulated FeSe₂ (NC-FeSe₂) with a controlled porous structure using a straightforward salt template-assisted strategy^[51]. As an SIB anode, NC-FeSe₂ achieved a reversible capacity of 520.4 mA h g^-1 over 100 cycles at 1 A g^-1. Even at 10 A g^-1, it maintained a capacity of 333 mAh g^-1 over 1,800 cycles, demonstrating exceptional long-cycle stability. The high ICE (78.9 %) and excellent diffusion kinetics are ascribed to its hierarchical porous structure and intrinsic pseudocapacitive behaviors^[51]. Park et al. synthesized a FeSe₂-amorphous carbon (FeSe₂-AC) composite anode for SIBs via a large-scale spray-drying strategy [Figure 6A]^[52]. As displayed in Figure 6B and C, ultrafine FeSe₂ nanoparticles are uniformly embedded in AC microspheres formed from a 30 g L^-1 dextrin spray solution. TEM images in Figure 6D and E reveal that the FeSe₂ crystals (< 20 nm) are homogeneously distributed within the carbon matrix. As shown in Figure 6F, increasing the dextrin concentration from 0, 10, 20 to 30 g L^-1 resulted in progressively improved discharge capacities of 85, 194, 203, 379 mAh g^-1 over 150th cycles at 0.5 A g^-1, respectively. The optimized FeSe₂-AC composite achieved stable reversible capacities of 431, 399, 375, 346, 326, 309, and 286 mA h g^-1 at current densities ranging from 0.2 to 5 A g^-1 [Figure 6G]^[52].

Figure 6. (A) Schematic illustration of the preparation process of the FeSe₂-AC composite powder by the spray drying process, (B and C) SEM images and (D and E) TEM images of FeSe₂-AC; (F) cycling performance of FeSe₂-AC at 0.5 A g^-1; (G) cycling performance of FeSe₂-AC at 0.5 A g^-1[52]. Copyright 2020, Elsevier.

Li et al. discovered that surface oxides significantly affect the electrochemical activity of FeSe₂^[53]. FeSe₂ with a larger size and lower oxide content demonstrated superior properties compared to FeSe₂/graphene composites with higher oxide content. By modulating the oxide content of the FeSe₂/graphene anode, a specific capacity of 459 mAh g^-1 was retained at 0.1 A g^-1. This composite also displayed excellent rate capability, with only a 10% capacity loss when the current density increased from 0.1 to 5 A g^-1. Remarkably, the FeSe₂/graphene maintained a capacity of 227 mAh g^-1 over 800 cycles even at 25 A g^-1[53]. The surface oxide was found to transform into a sodiated shell with poor conductivity and high mechanical strength, which induced phase-transfer resistance. This resistance hindered the sodiation of the FeSe₂ core and impeded the diffusion of Na⁺/electrons during subsequent sodiation processes. Zheng et al. reported that FeSe₂@NC micro rods, consisting of a FeSe₂ core and a NC shell with ample void space, were adopted as anodes for SIBs^[53]. This ingeniously designed porous core-shell structure enhanced Na⁺/electron transport, prevented FeSe₂ degradation from electrolyte exposure, and accommodated volume changes due to sufficient void space. As a result, the FeSe₂@NC microrods achieved outstanding Na⁺ storage performance, maintaining a capacity of 411 mAh g^-1 at 10.0 A g^-1 and 401.3 mAh g^-1 over 2,000 cycles at 5.0 A g^-1[53].

To maintain structural integrity during cycling, Pan et al. proposed a FeSe₂@NC composite anode. The strong interaction between FeSe₂ nanorods and NC facilitates fast interfacial Na⁺/e^- diffusion and enhances structural stability^[54]. As a result, the FeSe₂@NC electrode exhibited a capacity of 379.2 mA h g^-1 at a high current density of 10 A g^-1. When paired with a Na₃V₂(PO₄)₂F₃@reduced graphene oxide cathode, the full cell achieved 167.2 mAh g^-1 at 1 A g^-1 over 1,000 cycles^[54]. Zhou et al. synthesized a porous NC framework (NCF) embedded with FeSe₂ (P-FeSe₂/NCF) via a spray-drying and salt-template strategy^[55]. This unique structure facilitates Na⁺/electron transport and alleviates large volume changes during cycling. As an anode for SIBs, P-FeSe₂/NCF delivered 507 mAh g^-1 over 200 cycles at 2 A g^-1 and retained 386.7 mAh g^-1 over 500 cycles at 10 A g^-1[55]. Men et al. fabricated a CNT/FeSe₂/C composite by encapsulating FeSe₂/C within a carbon nanotube (CNT) framework through a facile wet-chemistry procedure followed by selenization^[56]. The resulting three-dimensional network enhanced electronic/ionic transport while providing void space to accommodate volume expansion. The CNT/FeSe₂/C composite exhibited excellent Na⁺ storage performance, delivering 546 mAh g^-1 over 100 cycles at 0.1 A g^-1. Even at a high mass loading of 16.9 mg cm^-2, it achieved impressive volumetric and areal capacities of 158 mAh cm^-3 and 5.06 mAh cm^-2, respectively^[56]. Furthermore, Yousaf et al. developed a FeSe₂-HGCNS composite using hollow graphitic carbon nanoshells, prepared via a spray granulation approach followed by selenidation^[57]. Benefiting from its distinctive 3D structure, the FeSe₂-HGCNS electrode maintained a capacity of 425 mAh g^-1 at 0.5 A g^-1 over 100 cycles^[57].

The sodium storage mechanism of Fe₇Se₈ proceeds through the following electrochemical reactions during discharging^[58-63]:

The corresponding charging reactions are as follows:

Tian et al. fabricated a monolithic architecture composed of radially aligned Fe₇Se₈ nanorod bundles (Fe₇Se₈ NRBs) using a facile self-template strategy followed by annealing^[58]. The Fe₇Se₈ NRB anode delivered a specific capacity of 300 mAh g^-1 at 0.5 A g^-1 and retained 190 mAh g^-1 over 8,000 cycles at a high current density of 20 A g^-1, indicating prominent long-cycling stability. This feasible strategy highlights the potential of structural engineering to enhance the electrochemical performance of selenide-based materials^[58]. Liu et al. developed a one-step pyrolysis strategy to prepare metal chalcogenide materials^[59]. The obtained carbon-coated Fe₇Se₈@C nanorods exhibited specific capacities of 442.4, 409.0, 393.8, 372.0, 365.3, and 353.8 mAh g^-1 at 442.4, 409.0, 393.8, 372, 365.3, and 353.8 mAh g^-1, respectively. The remarkable rate performance of the Fe₇Se₈@C anode is attributed to the carbon coating, which enhances overall conductivity and mitigates volume expansion during charging/discharging. To further address volumetric strain and improve structural stability, Chen et al. proposed an innovative stress release strategy by encapsulating yolk-shell Fe₇Se₈@C structures with MoSe₂ nanosheets^[60]. Due to its unique microstructure, the Fe₇Se₈@C@MoSe₂ composite exhibited an irreversible capacity of 473.3 mAh g^-1 at 0.1 A g^-1, maintained 274.5 mAh g^-1 at 5.0 A g^-1, and demonstrated 87.1% capacity retention over 600 cycles at 1.0 A g^-1. Chen et al. synthesized core-shell Fe₇Se₈ nanorods (Fe₇Se₈@NC) through an in-situ self-polymerization followed by a carbonization-selenization process^[60]. The resulting Fe₇Se₈@NC electrode maintained a capacity of 290.7 mAh g^-1 at 10 A g^-1 and exhibited 84.6 % capacity retention over 6,000 cycles at 5 A g^-1[60]. The core-shell configuration significantly enhanced Na⁺ diffusion and minimized irreversible side reactions, effectively accommodating volume changes and preserving structural integrity. Yuan et al. synthesized Fe₇Se₈@C nanotubes using Fe₂O₃ as the precursor and Fe₃O₄@C as a sacrificial template [Figure 7A]^[61]. As displayed in Figure 7B-D, the resulting Fe₇Se₈@C structures successfully retained the nanotube morphology of both the Fe₂O₃ precursor and Fe₃O₄@C intermediate. Utilized as an anode for SIBs, the Fe₇Se₈@C nanotubes demonstrated a capacity of 319 mAh g^-1 over 720 cycles at 2 A g^-1 [Figure 7E]. The excellent electrochemical performance is attributed to the hierarchical nanotube structure, which alleviates volume strain and provides efficient Na⁺/electron transport channels. Additionally, the NC shell enhances overall conductivity and helps maintain electrode integrity during cycling^[61,62].

Figure 7. (A) Schematic illustration of the preparation process of Fe₇Se₈@C. SEM images of (B) Fe₂O₃, (C) Fe₃O₄@C, and (D) Fe₇Se₈@C. (E) Cycling performance of Fe₇Se₈@C at 2.0 A g^-1[61]. Copyright 2022, Elsevier.

Yang et al. developed Fe₇Se₈@NC nanoboxes (Fe₇Se₈@NC NBs) by fully encapsulating Fe7Se8 within a robust NC shell via a facile etching and selenization process^[63]. As an anode material for SIBs, the Fe₇Se₈@NC NBs reached a Na⁺ storage capacity of 385.5 mAh g^-1 at 0.1 A g^-1 and maintained 345.2 mAh g^-1 at 1 A g^-1, exhibiting negligible capacity decay over 1,000 cycles^[63]. Wang et al. explored a carbon-coated Fe₇Se₈ nanosheet/nanorod composite (Fe₇Se₈@C/CNT-EG) through a one-step gas-phase pyrolysis method using expanded graphite (EG), selenium, and ferrocene as precursors^[64]. The Fe₇Se₈@C/CNT-EG composite, featuring a 3D conductive network with abundant mesopores and a large surface area, exhibited enhanced Na⁺ storage performance in an ester-based electrolyte. Notably, it demonstrated excellent cycling stability, retaining 97.5% of its capacity after 1,000 cycles at 1.0 A g^-1, and delivered a high rate capability of 325 mAh g^-1 at 2.0 A g^-1[63]. Wang et al., fabricated a hybrid material consisting of Fe₇Se₈ embedded in NC nanofibers (Fe₇Se₈/N-CNFs) using a facile electrospinning technique^[64]. As a SIB anode, the Fe₇Se₈/N-CNFs retained a high capacity of 286.3 mAh g^-1 at an ultrahigh current density of 20 A g^-1. The synergistic interaction between the Fe₇Se₈ nanoparticles and the cross-linked N-CNFs generated numerous defects and active Na⁺ storage sites, shortened the Na⁺ diffusion length, improved electrolyte infiltration, enhanced overall conductivity, and facilitated efficient electron/Na⁺ transport, thereby promoting superior reaction kinetics^[65].

Iron-based phosphides

Unlike iron oxides, which produce Na₂O (433 K, 5 × 10^-8 S cm^-1) during Na⁺ insertion and exhibit low conductivity, iron phosphates react with Na⁺ to form Na₃P (room temperature, > 1 × 10^-5 S cm^-1), thereby helping maintain electrochemical activity^[66-70]. The Fe-P bond features unbound lone pairs of electrons in the 3p and 3d orbits, which significantly enhance the localized charge density, improving electronic conductivity^[69-72]. Additionally, the Fe-P bond lengths, ranging from 2.186 to 2.447 Å, facilitate Na⁺ migration within the FeP crystal structure, improving ionic conductivity and partially mitigating volumetric strain. The Na⁺ storage process in FeP is as follows^[66-70]:

However, challenges such as volume expansion during charging/discharging and limited electronic conductivity hinder large-scale application. Therefore, Yang et al. designed a FeP/graphite composite anode through a straightforward ball milling strategy^[66]. This composite significantly enhanced conductivity and achieved a capacity of 134 mAh g^-1 at 0.5 A g^-1[66]. To overcome Fe⁰ agglomeration and poor conductivity in FeP, Xu et al. in situ encapsulated ultrafine FeP particles in porous carbon (FeP@C) via a MOF-derived phosphorization strategy [Figure 8A]^[67]. Energy-dispersive X-ray (EDX) elemental mapping signals [Figure 8B-E] indicate the co-existence of Fe, P, and C, with FeP particles fully wrapped in the carbon framework. As a Na⁺ storage anode, the FeP@C electrode achieved capacities of 408, 366, 295, 235, 173, and 107 mAh g^-1 at 0.1, 0.2, 0.5, 1.0, 2.0, and 4.0 A g^-1, respectively [Figure 8F]. Notably, it maintained 190 mAh g^-1 over 500 cycles at 1.0 A g^-1 [Figure 8G]^[67]. This outstanding cycling performance is attributed to the MOF-derived FeP@C, which suppresses Fe coarsening, improves conductivity, and buffers the volumetric strain. Li et al. further improved performance by encapsulating FeP nanodots within NC nanosheets to create an open-framework FeP@NC structure with high porosity and surface area^[68]. Used as SIB anodes, FeP@NC delivered a capacity of 374 mAh g^-1 over 2,000 cycles at 0.5 A g^-1[68]. This exceptional cycling stability is credited to the rationally designed hierarchical structure, which preserves integrity and promotes fast reaction kinetics. Wang et al. synthesized a hollow FeP@carbon structure wrapped in graphene (GR) (H-FeP@C@GR) via a hydrothermal method followed by carbon coating and phosphidation^[69]. This anode retained a capacity of 400 mAh g^-1 over 250 cycles at 0.1 A g^-1. The 3D hierarchical architecture endowed open pathways for electron/Na⁺ transport and enhanced reaction kinetics. Moreover, the carbon shell facilitated the formation of a stable SEI layer, suppressing side reactions^[69]. Jiang et al. fabricated FeP nanoparticles embedded in N,P-doped carbon frameworks (FeP@PNC) using a phytic acid-derived self-assembly process^[70]. This anode holds a high reversible capacity of 340.1 mA h g^-1 at 0.1 A g^-1 and remarkable cycling capability, maintaining 224.5 mA h g^-1 at 0.5 A g^-1 over 4,500 cycles. These superior electrochemical properties stem from the unique 3D heterostructure, which enhances structural stability, accelerates reaction kinetics, and prevents nanoparticle aggregation during sodiation/desodiation.

Figure 8. (A) Schematic illustration of the preparation process of FeP@C. (B) Dark-field TEM image; (C-E) corresponding EDX mapping signals of Fe (C), P (D), and C (E) elements. (F) Rate performance of FeP@C measured from 0.1 to 4.0 A g^-1 and (G) cycling performance of FeP@C at 2.0 A g^-1[67]. Copyright 2019, Elsevier.

Compared to conventional power-based anodes, self-supported electrodes are typically grown on a conductive metal/carbon matrix, which can mitigate the side effects of binders and enhance the energy density of SIBs. Furthermore, such binder-free electrodes are intrinsically flexible, ultrathin, lightweight, and bendable, allowing them to maximize space utilization in electronic devices. For instance, Shi et al. reported a free-standing FeP@NPC film anode for SIBs, where FeP nanoparticles were embedded in a 3D cross-linked N,P-doped carbon (NPC) fiber network^[71]. The FeP nanoparticles were in situ confined within the N,P-doped carbon fibers via thermal treatment of PAN nanofibers. The 3D-connected NPC framework offered efficient electron/ion transport pathways and featured void spaces to accommodate volumetric strain during Na⁺ extraction/insertion. This FeP@NPC anode achieved a capacity of 557 mAh g^-1 at 0.1 A g^-1 over 1,000 cycles. Their work broadens the approach to fabricating binder-free phosphide anodes for flexible SIBs^[71]. Similarly, Park et al. developed a self-supported film composed of superfine FeP@C particles dispersed on P-doped graphene (FeP@C@PG) through solvent-free pyrogenic decomposition followed by roll pressing^[72]. PA served as both the phosphorus and carbon source, enabling in-situ confinement of ultrafine FeP particles within the P-doped carbon matrix. This hierarchical structure accelerated diffusion kinetics, mitigated significant changes, and suppressed the aggregation/pulverization of FeP particles during charging/discharging. As a result, the FeP@C@PG film exhibited a capacity of 536.6 mAh g^-1 over 1,000 cycles at 1 A g^-1, and a high rate capability of 440.7 mAh g^-1 at 5 A g^-1.

Compared with other oxides and sulfides, iron-based phosphide materials have a lower charging/discharging potential, making them more suitable as SIB anodes. If their electrochemical properties can be further improved, they hold great promise for future applications.

Iron-based fluorides

Iron-based fluorides can support multi-electron transfer through multi-step electrochemical reactions, enabling them to reach high specific capacities^[73-77]. Common iron-based fluorides include FeF₂, FeF₃, and hydrated forms. For instance, FeF₃ exhibits an ultrahigh theoretical capacity of 712 mAh g^-1 along with a relatively high operating voltage, which attracted considerable attention for its potential as a cathode material in SIBs^[77-82]. The Na⁺ storage mechanism of FeF₃ is as follows^[77-82]:

At present, the electrochemical performance of fluoride materials remains unsatisfactory due to significant voltage hysteresis and sluggish reaction kinetics, resulting in rapid capacity decay^[74,80-82]. Therefore, developing iron-based fluoride cathodes for SIBs and enhancing their electrochemical properties is of great importance.

One major challenge is the severe pulverization of iron fluorides during cycling, which results in rapid structural degradation and capacity fading. To address this, Zhang et al. designed an amorphous FeF₃/C nanocomposite cathode using Fe-MOF nanorods as precursors via high-temperature carbonization and fluorination^[74]. This FeF₃/C cathode demonstrated excellent cycling stability, retaining 126.7 mAh g^-1 over 100 cycles at 75 mA g^-1[74]. The improved performance is ascribed to the amorphous structure and the porous carbon backbone formed from the carbonized organic ligands of the Fe-MOF, which facilitate Na⁺/electron diffusion and enhance reaction kinetics. Sun et al. developed a free-standing FeF₃ cathode and employed sodium-difluoro(oxalate)borate (NaDFOB) as the electrolyte salt for SIBs [Figure 9A]^[75]. XRD analysis [Figure 9B] confirmed that the synthesized material matched the hexagonal FeF₃ phase. SEM images [Figure 9C and D] reveal that FeF₃ nanoparticles were uniformly distributed within or on the surface of the NFs. TEM images [Figure 9E] further show that FeF₃ crystals are well- encapsulated within the carbon matrix. As shown in Figure 9F, cells using NaDFOB demonstrated superior cycling stability compared to those using NaPF₆ or NaClO₄ over 120 cycles. Moreover, as illustrated in Figure 9G, cells using a DEC/EC/DMC solvent mixture achieved the highest capacity retention of 69.4%, significantly outperforming those using EC/DEC (28.17%) or EC/DMC (31.4%). This enhanced performance is associated with the synergistic effect between the nanoconfined FeF₃ cathode and the NaDFOB electrolyte. Quantum mechanical calculations revealed that DFOB^- facilitated the formation of a thin CEI layer on the FeF₃-CNFs, improving stability^[75]. Despite these advancements, the practical application of FeF₃-based cathodes remains hindered by low capacity utilization and poor cycling performance. Sun et al. investigated various ionic liquid (IL) electrolytes to enhance the Na⁺ storage performance of FeF₃ across a wide temperature range^[76]. They found that the Pyr_1,4FSI electrolyte exhibited an ignorable decay rate of ~0.023% per cycle over 1,000 cycles. Pyr_1,3FSI showed better stability at elevated temperatures. A protective and ion-conductive CEI film was formed during cycling in ILs, suppressing side reactions, especially at high temperatures^[76].

Figure 9. (A) Schematic of the preparation procedures of FeF₃-C NFs and FeF₃-C electrode cycled in the NaDFOB electrolyte; (B) XRD pattern of FeF₃-C NFs cathode; (C and D) SEM images of FeF₃-CNFs; (E) High-resolution TEM image of FeF₃-CNFs. (F) Cycling performance of FeF₃-CNFs with different sodium salts and (G) cycling performance of FeF₃-CNFs in 1M NaDFOB electrolyte with different solvents^[75]. Copyright 2020, Royal Society of Chemistry.

Ma et al. proposed a FeF₃-Fe-RGO composite as a cathode for SIBs, in which a conductive matrix of metallic Fe and RGO was formed in situ from a single FeF₂ grain during electrochemical cycling^[77]. This FeF₃-Fe-RGO cathode delivered a capacity of 150 mAh g^-1 at 0.05 A g^-1 and maintained a specific capacity of 60 mAh g^-1 even at 2.0 A g^-1[77]. The superior electrochemical performance is attributed to the enhanced electronic conductivity provided by the RGO and metallic Fe-based conductive framework. This design strategy could be extended to other electrode materials that offer high capacity but suffer from low electrical conductivity.

FeF₂ has also emerged as a promising conversion-type cathode material for SIBs. Accordingly, Maulana et al. synthesized FeF₂ nanoparticles encapsulated in N-doped graphitic carbon (FeF₂@NGC) for use as a SIB cathode^[78]. The optimized FeF₂@NGC delivered a high reversible capacity of 214.2 mAh g^-1 over 500 cycles at 300 mA g^-1, with a low capacity fading rate of just 0.042% per cycle, indicating excellent long-term stability^[78]. Similarly, Ni et al. prepared FeF₂@mesoporous hollow carbon spheres (FeF₂@MHCS) as cathodes for SIBs^[79]. The hierarchical structure of FeF₂@MHCS suppressed agglomeration, facilitated Na⁺/electron diffusion, and mitigated the volume variation during cycling. This cathode achieved a capacity of 220 mAh g^-1 at 0.03 A g^-1 and 64 mA h g^-1 at 10 A g^-1, and retained 112 mA h g^-1 over 100 cycles at 0.2 A g^-1[79]. In another study, Ni et al. fabricated a FeF₂-RGO nanocomposite cathode using poly(acrylic acid) (PAA) as a binder for SIBs^[80]. The use of PAA was found to stabilize the electrode structure, improve the utilization of active materials, and significantly enhance Na⁺ storage performance. The FeF₂-RGO cathode achieved superior cycling stability (175 mAh g^-1 at 0.2 A g^-1) and good rate capability (78 mAh g^-1 at 10 A g^-1)^[80].

Despite these advancements, the poor electronic conductivity and side reactions of FeF₃ cathodes result in sluggish reaction kinetics and rapid capacity decay during cycling, impeding their large-scale application. To address this, Liu et al. employed a surface modification method, synthesizing hollow FeF₃·0.33H₂O microspheres through a solvothermal approach followed by coating with AlPO₄^[81]. This surface treatment significantly reduced charge-transfer resistance and improved Na⁺ diffusion. Compared to pristine FeF₃·0.33H₂O, the AlPO₄-coated version (with 4 wt.% AlPO₄) displayed an initial capacity of 290 mAh g^-1 and retained 211 mAh g^-1 over 80 cycles within a voltage window of 1.2~4.0 V^[79]. The enhanced electrochemical properties were ascribed to the hierarchical porous structure that promotes electrolyte permeation and rapid Na⁺/electron transport. Moreover, the multifunctional AlPO₄ coating enhanced overall conductivity, reduced volumetric strain, and suppressed surface side reactions during cycling.

In summary, coupling iron-based fluoride nanoparticles with conductive carbon materials can effectively improve electrical conductivity, alleviate volume expansion and particle agglomeration, and enhance the overall electrochemical properties of SIB cathodes.

Other iron-based materials

Other iron-based materials - such as FeTiO₃, FeTe₂, FePS₃, FePO₄, and Fe₂(SO₄)₃ - have also shown promise for application in SIBs due to their relatively high theoretical specific capacities^[82-87]. Researchers have explored strategies to control the growth and microstructure of these materials to improve their cycling stability and rate performance. For example, two-dimensional (2D) FePS₃ nanosheets, a typical ternary metal phosphosulfide, were synthesized via ultrasonic exfoliation. A FePS₃ nanosheets@MXene composite was subsequently fabricated by in situ mixing of the exfoliated nanosheets with ultrathin MXene layers^[83]. This hybrid material improved electronic conductivity and increased the specific surface area, thus accelerating Na⁺ diffusion and alleviating volumetric strain during repeated cycling. As a result, the FePS₃ nanosheets@MXene composite reached a reversible capacity of 676.1 mAh g^-1 over 90 cycles at 0.1 A g^-1[83]. This work provides a valuable pathway for the development of 2D/2D conversion-type electrode materials with enhanced performance for SIBs. In addition, tellurium-based materials have attracted attention due to tellurium’s inherently high electrical conductivity. Iron tellurides, in particular, have been investigated for Na⁺ storage applications. Recently, FeTe₂ nanoparticles embedded in nitrogen-doped carbon nanofibers (FeTe₂/CNFs) were synthesized via electrospinning^[84]. The FeTe₂/CNF anode demonstrated an initial capacity of 406.8 mAh g^-1 and retained 221.3 mAh g^-1 over 1,000 cycles at 1 A g^-1. These outstanding electrochemical properties are attributed to the synergistic interaction between the FeTe₂ particles and the carbon nanofiber matrix, which buffers volume changes and enhances structural stability during cycling^[84]. FePO₄, with a high theoretical capacity of 175 mAh g^-1, can be easily synthesized on a large scale through a facile precipitation procedure at room temperature, making it a promising cathode candidate for SIBs^[88]. Furthermore, monoclinic Fe₂(SO₄)₃, when coupled with a Na anode, exhibits an average operating voltage of ~3.25 V and can store 1.8 Na⁺, delivering a specific capacity of ~120 mAh g^-1, thus emerging as another potential cathode material^[89].

Iron-based materials offer several advantages, including abundant raw resources and low production costs. A wide variety of iron-containing compounds and metal composites have been explored as electrode materials, leading to significant advancements in the development of both cathode and anode materials for SIBs.