Recent Progress on Synthesis and Electrochemical Performance of Iron Fluoride Conversion Cathodes for Li-Ion Batteries

Abstract

Despite notable advancements in lithium-ion battery (LIB) technology, growing industrialization, rising energy demands, and evolving consumer electronics continue to raise performance requirements. As the primary determinant of battery performance, cathode materials have become a central research focus. Among emerging candidates, iron-based fluorides show great promise due to their high theoretical specific capacities, elevated operating voltages, low cost (owing to abundant iron and fluorine), and structurally diverse crystalline forms such as pyrochlore and tungsten bronze types. These features make them strong contenders for next-generation high-energy, low-cost LIBs. This review highlights recent progress in iron-based fluoride cathode materials, with an emphasis on structural regulation and performance enhancement strategies. Using pyrochlore-type hydrated iron trifluoride (Fe₂F₅·H₂O), synthesized via ionic liquids like BmimBF₄, as a representative example, we discuss key methods for tuning physicochemical properties—such as electronic conductivity, ion diffusion, and structural stability—via doping, compositing, nanostructuring, and surface engineering. Advanced characterization tools (XRD, SEM/TEM, XPS, Raman, synchrotron radiation) and electrochemical analyses are used to reveal structure–property–performance relationships. Finally, we explore current challenges and future directions to guide the practical deployment of iron-based fluorides in LIBs. This review provides theoretical insights for designing high-performance, cost-effective cathode materials.

1. Introduction

In the pursuit of high-energy-density energy storage systems, iron-based fluoride cathode materials have attracted significant attention due to their remarkable theoretical capacity of 712 mAh·g⁻¹ and high operating voltage of 2.74 V [1]. These conversion–reaction-based materials demonstrate tremendous potential to break through the energy density limits of current lithium-ion batteries (LIBs). However, the path from laboratory research to practical applications is fraught with multiple challenges. The strong ionic bonding characteristics of iron fluoride result in extremely low intrinsic conductivity (<10⁻¹⁰ S/cm), which not only causes severe electrode polarization and voltage hysteresis but also leads to less than 50% utilization of active materials [2,3]. More critically, the dramatic volume changes of up to 200% during charge/discharge processes trigger particle fracture and conductive network collapse, while the irreversible decomposition of conversion product LiF and the agglomeration of Fe nanoparticles further accelerate capacity fading [4]. These interconnected issues collectively form formidable barriers that must be overcome for the practical application of iron fluoride.

To address these challenges, researchers have developed a series of innovative solutions, and the development history of iron fluoride modified materials is shown in Figure 1. By precisely controlling the nanoscale structural features of materials—such as constructing hollow nanospheres or graphene-wrapped nanowires—not only is conductivity significantly improved, but the damage caused by volume expansion is also effectively mitigated [5]. In the realm of crystal engineering, the successful preparation of dehydrated HTB-phase and pyrochlore structures provides efficient three-dimensional transport channels for lithium ions [6]. More groundbreaking is the precise doping of elements like Co/O, which enables the regulation of electronic structure and reduces voltage hysteresis to as low as 0.27 V [7]. In the field of interface engineering, technologies such as graphitic carbon coating and TiO₂ layers have significantly enhanced interfacial stability [8]. These multi-scale modification strategies work synergistically to comprehensively advance the performance of iron fluoride materials.

This review aims to systematically summarize these critical breakthroughs and provide an in-depth analysis of how various modification strategies address the key challenges of iron fluoride from different dimensions. Special attention is given to key scientific issues such as the synergistic construction mechanism of nanoconfinement effects and conductive networks, the influence of crystalline water content on lithium transport channels, and the regulatory role of doping elements in the reversibility of conversion reactions. By elucidating the intrinsic structure–performance relationship between, this review not only charts a course for the practical development of iron fluoride cathodes but also provides valuable insights for the design of other conversion-type electrode materials. Against the backdrop of energy transition and carbon neutrality, these research advances hold significant theoretical and practical importance for developing next-generation high-energy-density LIBs.

2. Cathode Materials for Lithium-Ion Batteries

Currently, commercially cathode materials such as LiCoO₂, LiFePO₄ and NCM have enabled widespread application of LIBs, while revealing several critical limitations due to the increased demand in various applications [17]. Firstly, their relatively high atomic mass severely restricts the overall energy density. In high-energy-demand applications like long-range electric vehicles and large-scale energy storage systems, insufficient energy density forces batteries to require more space and weight to meet operational requirements, thus reducing equipment efficiency and increasing transportation/ installation costs [18]. Secondly, their operating voltages are approaching the limits of electrochemical batteries, making further significant voltage enhancement technically challenging under current frameworks. This limitation prevents effective improvement of output power, restricting applications in fast-charging devices and high-performance electronics that require high power, while also hindering expansion into emerging fields. Thirdly, high raw material costs maintain elevated production expenses that ultimately affect market prices, creating a major obstacle for widespread adoption. Experimental data clearly demonstrates that cathode materials provide far greater improvement to total power density than anode materials at equivalent specific capacity increases, further highlighting the importance of developing new cathode materials [19,20,21].

Therefore, developing novel lithium-ion battery cathode materials has become crucial for performance enhancement and application expansion. Based on differences in electrochemical reaction mechanisms, current cathode materials can be classified into intercalation-type and conversion-type [22], with in-depth research on both offering new opportunities for lithium-ion battery development.

2.1. Intercalation-Type Cathode Materials

The cathode materials used in commercial LIBs are predominantly intercalation-type materials. During the charging process, complex yet orderly electrochemical reactions occur within the battery [23]. Driven by the potential difference, lithium ions (Li⁺) deintercalate from the crystal lattice structure of the cathode material and migrate through the electrolyte toward the anode during charge. The electrolyte serves as an ionic conduction medium, providing pathways for lithium ion transport, while the separator functions to isolate the cathode and anode with precise ion-selective properties—allowing only lithium ions to pass through—to maintain normal internal battery reactions. Ultimately, the lithium ions intercalate into the anode material. Simultaneously with Li⁺ transfer, an equivalent amount of charge moves to the anode via the external circuit to maintain charge balance in the battery system. During discharge, the movement of ions and electrons reverses compared to charging; lithium ions deintercalate from the anode and return to the cathode via the same path, while electrons flow back to the cathode through the external circuit, generating current to power external devices. Based on significant differences in their microscopic structures, intercalation-type materials can be broadly classified into three major categories: layered, spinel, and polyanionic compounds [24,25]. Research has primarily focused on a few specific materials, as shown in

Table 1. Summary of Currently Common Embedded Cathode Materials. Reproduced with permission. Copyright 2014, Royal Society of Chemistry [31]. Reproduced with permission. Copyright 2019, Royal Society of Chemistry [32].

Name	Lithium Cobalt Oxide (LCO)	Nickel Cobalt Manganese (NCM)	Lithium Manganese Oxide (LMO)	Lithium Iron Phosphate (LFP)
Chemical Formula	LiCoO2	LiNixCoyMn1−x−yO2	LiMn2O4	LiFePO4
Crystal Structure [31,32]
Structure	Layered	Layered	Spinel	Olivine
Theoretical Capacity (mAh·g−1)	274	273–285	148	170
Practical Capacity (mAh·g−1)	135–150	155–220	100–120	130–140
Li+ Diffusion Coefficient (cm2/s)	10−12–10−11	10−11–10−10	10−14–10−12	10−16–10−14
Conductivity (S/cm)	10−3	10−5	10−5	10−9
Advantages [33]	High voltage plateau, simple synthesis, good cycling stability [34]	Stable high discharge voltage, high energy density and capacity [35]	Good rate capability, low cost, environmentally friendly [36,37]	Low cost, excellent cycling and thermal stability [38], eco-friendly [39]
Disadvantages	Low Li+ utilization, environmental pollution [40], poor safety	Li/Ni cation mixing at high voltage leads to poor cycling, low electronic conductivity [41]	Low capacity, poor cycling stability [42,43]	Poor low-temperature performance, low conductivity, slow Li+ diffusion [39,44]

, including layered lithium cobalt oxide (LiCoO₂) and lithium nickel cobalt manganese oxide (LiNi_xCo_yMn_1−x−yO₂) [26], spinel-type lithium manganese oxide (LiMn₂O₄) [27,28], and olivine-type lithium iron phosphate (LiFePO₄) [29]. Layered lithium cobalt oxide (LiCoO₂), with its relatively high operating voltage and excellent charge/discharge performance, has been widely used in early LIBs, particularly dominating applications in small electronic devices requiring high energy density, such as mobile phones and tablets [30]. Lithium nickel cobalt manganese oxide (LiNi_xCo_yMn_1−x−yO₂), by adjusting the ratios of nickel, cobalt, and manganese, achieves a favorable balance between energy density, cycling stability, and safety, gradually becoming a popular cathode material choice for electric vehicles and similar applications. Spinel-type lithium manganese oxide (LiMn₂O₄) offers advantages such as low cost, abundant resources, and environmental friendliness. However, it suffers from manganese dissolution during cycling, leading to battery performance degradation—an issue researchers are actively working to address. Olivine-type lithium iron phosphate (LiFePO₄) has garnered significant attention for its high safety, long cycle life, and stable structure, demonstrating unique advantages in energy storage systems and applications with stringent safety requirements. Nevertheless, its relatively low electronic conductivity limits its further expansion into high-power applications.

However, constrained by the inherent crystal structure characteristics of these materials, there exists an upper limit to the saturation intercalation degree of lithium ions in these intercalation-type materials [25]. This limitation can be linked to a container with finite capacity that cannot indefinitely accommodate lithium ions. Such restriction directly results in a limited number of lithium ions that can be deintercalated and reintercalated during a single charge–discharge cycle, consequently leading to relatively low mass-specific capacity of the batteries. As demands for battery energy density are constantly escalating, existing intercalation-type materials based on lithium deintercalation/intercalation mechanisms are struggling to meet the growing energy density requirements. This situation urgently calls for researchers to explore and develop entirely new electrode material systems, which may bring groundbreaking advancements to lithium-ion battery technology [22].

2.2. Conversion-Type Cathodes

In the ongoing exploration and innovation of lithium-ion battery technology, a novel class of cathode materials based on reversible chemical conversion reactions for electron transfer has emerged in recent years, rapidly garnering widespread attention from both the scientific community and industry [45,46]. Unlike conventional lithium-ion battery cathode materials that primarily rely on intercalation/deintercalation mechanisms, these new conversion-type cathode materials represent a distinct approach. During electrode reactions, these materials can fully exploit the potential of their various oxidation states to achieve efficient exchange of all electrons. This characteristic enables them to demonstrate significantly higher discharge capacities than traditional lithium-ion intercalation/deintercalation reactions [47,48], earning them recognition as next-generation cathode materials. This reaction can be represented as Equation (1) [49]:

M_aX_b + (b·n) Li ⇄ aM + bLi_nX

(1)

where M represents transition metal cations, which may include Fe³⁺, Bi³⁺, V³⁺, Ti³⁺, Co²⁺, Ni²⁺, etc., and X represents anions, which may include F⁻, O²⁻, S²⁻, N³⁻, P³⁻. The chemical conversion reaction mechanism exhibits significant differences from the traditional lithium intercalation mechanism. Firstly, from the perspective of structural changes: During lithium intercalation, lithium ions insert into the crystal lattice of the electrode material, causing a certain degree of lattice expansion and slight distortion, while the main structure of the electrode material generally remains intact. These structural changes are reversible and can maintain relative stability over multiple charge–discharge cycles. In contrast, electrochemical conversion reactions display completely different structural change characteristics. During discharge, the metal ions in the electrode material undergo displacement reactions with lithium, causing the original crystal structure to completely collapse and form nanoscale metal particles and lithium compounds. This structural transformation is irreversible and leads to significant changes in the overall structure of the electrode material [50]. Secondly, in terms of lithium storage capacity, lithium intercalation reactions, constrained by the crystal structure, typically involve electron transfer of less than one unit, resulting in relatively limited capacity. Nevertheless, lithium intercalation reactions demonstrate excellent cycling performance and rate capability, maintaining high-capacity retention over multiple charge–discharge cycles. In comparison, electrochemical conversion reactions are not restricted by the crystal structure and can achieve multi-electron transfer, thereby storing higher capacity. However, this high capacity is often accompanied by poor cycling performance and rate capability, as the electrode material is prone to structural damage during repeated charge–discharge cycles, leading to rapid capacity decay.

Transition metal oxides, fluorides, sulfides, phosphides, and nitrides can all achieve lithium storage through conversion reactions. However, with the exception of fluorides, these materials exhibit relatively low potential ranges, as shown in Figure 2, making them unsuitable for use as cathode materials [51]. The reason fluorides have emerged as key candidate cathode materials for high-capacity LIBs primarily lies in their unique chemical properties. The F⁻ ions in fluorides possess strong electronegativity, enabling them to form ionic bonds with transition metals M to create MF_n. Due to the high chemical bond energy between M-F, the discharge plateau of MF_n can be elevated into the operating voltage range of cathode materials, thereby exhibiting excellent electrochemical performance in LIBs. These characteristics make transition metal fluorides one of the most promising candidate cathode materials for high-capacity LIBs [51,52].

Thus, conventional intercalation cathodes face inherent limitations in capacity due to constraints in their crystalline structures. Conversion reaction mechanisms, driven by multi-electron transfer processes, offer a promising alternative, and iron fluoride materials (with FeF₃ as a representative example) have emerged as prime candidates due to their ultrahigh theoretical capacity (712 mAh·g⁻¹) [14]. This establishes the foundation for discussing their unique characteristics and modification strategies in the subsequent sections.

3. Conversion Mechanisms and Crystal Structure Engineering

3.1. Conversion Mechanism of Iron Fluoride

In the ongoing research of LIBs, metal fluorides have emerged as a research hotspot due to their unique advantages in battery performance. From a mechanistic perspective, these materials not only enable reversible lithium-ion intercalation and deintercalation—providing accommodation sites for Li⁺ during charge/discharge processes like some conventional cathode materials—but more importantly, can also store energy through chemical reactions with lithium [51]. This distinctive energy storage mechanism endows metal fluorides with significant advantages as cathode materials for LIBs in terms of energy density. To date, numerous transition metal fluorides have been reported, as shown in

Table 2. Metal fluoride conversion reaction data sheet [53].

MFn	Gf (kJ·mol−1)	Electromotive Force/V	Theoretical Capacity (mAh·g−1)
LiF	−589	—	0
TiF3	−1361	1.396	767
VF3	−1227	1.863	745
MnF2	−807	1.919	577
MnF3	−1000	2.647	719
FeF2	−663	2.664	571
FeF3	−972	2.742	712
BiF3	−902	3.124	302
CoF2	−627	2.854	553
CoF3	−719	3.617	694
NiF2	−604	2.964	554
CuF2	−492	3.553	528
ZnF2	−714	2.404	518
SnF2	−601	2.984	342
AgF	−187	4.156	211
PbF2	−617	2.903	218
CaF2	−1173	0.0259	686
BaF2	−1158	0.104	306

including CuF₂, TiF₂, BiF₂, CoF₂, NiF₂, CrF₂, FeF₂, and FeF₃, with many of them having far more theoretical capacity than conventional insertion-type cathode materials [53]. However, in practical applications, their electrochemical performance varies considerably due to differences in crystal structure, electron cloud distribution, and chemical activity arising from their distinct transition metal elements. For instance, materials such as NiF₂, CuF₂, CoF₂, and MnF₂ exhibit poor capacity retention during long-term cycling, making them unsuitable for high-performance lithium-ion battery applications [54,55,56,57].

In contrast, iron fluorides stand out among these transition metal fluorides and have become the most extensively studied, owing to their superior properties such as a high discharge potential (2.74 V), exceptional high-apacity performance, low cost, non-toxicity, and environmental friendliness [58]. Taking FeF₃ as an example, its theoretical electrode potential reaches 2.74 V. The reaction between FeF₃ and Li⁺ proceeds via a three-step mechanism, as shown in Equations (2)–(4) [59]:

FeF₃ + 0.5 Li+ + 0.5 e⁻ → Li_0.5FeF₃ (4.5–3.2V)

(2)

Li_0.5FeF₃ + 0.5 Li+ + 0.5 e⁻ → FeF₂ + LiF (3.2–2.5V)

(3)

FeF₂ + LiF + 2 Li+ + 2 e⁻ → Fe + 3LiF (2.5–1.5V)

(4)

The reaction mechanism proceeds as follows: (1) Li⁺ inserts into the FeF₃ lattice via a solid-solution reaction without phase transformation, involving a 0.5-electron transfer; (2) a pre-lithiation process occurs with an additional 0.5-electron transfer, leading to the formation of FeF₂; (3) a conversion reaction takes place at ~1.5 V vs. Li⁺/Li, involving a 2-electron transfer and complete crystal structure reconstruction to form new phases (Fe and LiF) [13,60]. This three-step reaction involves a total of 3-electron transfer, delivering a high theoretical capacity of 712 mAh·g⁻¹ [58].

As illustrated in Figure 3 [61], the initial lithiation process of FeF₃ involves a complex three-phase reaction mechanism. During this process, Li⁺ intercalates and migrates into the hexagonal close-packed (hcp) anion framework of FeF₃, reducing trivalent Fe³⁺ to divalent Fe²⁺ and forming an A-Li_xFe_1−δF₃ framework, while simultaneously generating FeF₂. The resulting FeF₂ may undergo lattice displacement, forming a tetragonally close-packed (tcp) anion sublattice. As the reaction progresses, A-Li_xFe_1−δF₃ transforms into B-Li_1+2δFe_1−ΔF₃ through continuous Li⁺ intercalation, during which Fe³⁺ is completely reduced to Fe²⁺. Subsequently, B-Li_1+2δFe_1−ΔF₃ undergoes further lithiation via a topotactic displacement process to form another intermediate phase C-Li_1+2ηFe_2+1−ηF₃, which maintains a similar structure to the B-phase. Ultimately, the C-phase Li₁₊₂ηFe_2+1−ηF₃ continues lithiation to form a composite of Fe and LiF. Notably, during this process, the F⁻ sublattice in LiF reverts from hcp to a face-centered cubic (fcc) structure. During the charging process, the initial delithiation of LiF consumes most of the Fe. After a series of charge reactions, approximately two-thirds of the Fe tends to form FeF₂, while the remaining one-third is reconverted back to the C-phase through a reverse pathway. In subsequent reactions, this portion of Fe undergoes lattice displacement and delithiation processes, successively transforming back into the B-phase and A-phase.

Despite its excellent theoretical performance, the practical electrochemical performance of FeF₃ is severely limited by its poor intrinsic conductivity. The strong ionic character of FeF₃ results in a wide band gap and inferior electronic conductivity, which significantly compromises its electrochemical activity during charge/discharge processes. In 1997, Arai et al. first reported the electrochemical activity of hexagonal FeF₃ [9]. However, when tested within a voltage range of 2.5–4.5 V at a current density of 2 A/m², its discharge capacity was only 80 mAh·g⁻¹—far below the theoretical value. A breakthrough came when Amatucci et al. pioneered the application of metal fluorides as cathode materials [10]. By employing high-energy ball milling to construct a nanostructured FeF₃-C composite, they achieved remarkable lithium storage capacity for FeF₃-based cathodes. Under optimized conditions (70 °C, voltage window: 1.5–4.5 V), the composite delivered an exceptional discharge capacity of 773 mAh·g⁻¹. This landmark work rapidly established iron-based fluorides as a research focus in lithium-ion battery cathode development.

3.2. Iron Fluorides with Different Crystal Structures

Iron-based fluorides exhibit diverse crystalline structures due to the variable valence states of iron and differences in lattice water content. The following polymorphs have attracted significant research attention: hexagonal tungsten bronze-type (HTB-FeF₃·0.33H₂O), rhenium trioxide-type (ReO₃-FeF₃), and pyrochlore-type (Pyrochlore-FeF₃·0.5H₂O).

(1) Hexagonal Tungsten Bronze-type Phase (HTB-FeF₃·0.33H₂O)HTB- FeF₃·0.33H₂O crystallizes in the orthorhombic system with Cmcm space group. As shown in Figure 4a,b [11], the structure consists of two distinct octahedra—Fe(1)F₆ and Fe(2)F₆—which share vertex F atoms to form interconnected hexagonal channels [11]. This unique architecture provides exceptional advantages for Li⁺ transport and storage, facilitating lithium ion migration. The lattice water molecules play dual critical roles: (i) Structural stabilization: they maintain framework integrity during Li⁺ intercalation/deintercalation, preventing FeF₃ structural collapse; (ii) Enhanced kinetics: their presence expands the tunnel dimensions, creating favorable diffusion pathways that significantly improve Li⁺ mobility and consequently optimize battery performance [11]. The electrochemical reaction of HTB-FeF₃·0.33H₂O is shown in Equations (5) and (6) as follows:

Li⁺ + FeF₃∙0.33H₂O + e⁻ → LiFeF₃∙0.33H₂O+ LiF (4.5−1.8 V)

(5)

LiFeF₃∙0.33H₂O + 2Li+ + 2e⁻ → LiF + Fe + 0.33H₂O (1.8−1.6 V)

(6)

(2) Rhenium trioxide-type Phase (ReO₃-FeF₃) adopts a distorted ReO₃-type trigonal structure (space group R3⁻m) [63,64]. As illustrated in (Figure 4c) [62], its crystal architecture resembles the ABX₃ perovskite type, featuring a three-dimensional tunnel structure formed by corner-sharing FeF_6/2octahedra. In this configuration, Fe³⁺ ions are stably located in the (102) planes of the rhombohedral structure and Li⁺ ions undergo intercalation/deintercalation through the A-site vacancies in the (204) planes [65]. This distinctive structural arrangement endows ReO₃-FeF₃ with promising potential for lithium-ion battery applications.

(3) The pyrochlore-type structure of FeF₃∙0.5H₂O is highly similar to the known crystal structure of AlF₃∙0.5H₂O, featuring a unique three-dimensional microporous framework. As illustrated in (Figure 4d), within this structure, each Fe atom coordinates with six F atoms to form an octahedral FeF₆ unit. These FeF₆ octahedra are interconnected by sharing a single F atom, constructing large hexagonal cavities—a structural motif analogous to those found in the FeF₃·0.33H₂O unit cell, which serves as the foundation for the 3D framework [66]. Furthermore, four closely packed FeF₆ octahedra combine to form a (FeF₆)₄ tetrahedral unit. These units vertically interlink with other (FeF₆)₄ tetrahedra, creating an expanded tetrahedral structure with a quadrilateral cavity at its center. This interlocking atomic arrangement ultimately forms a three-dimensional intersecting microporous framework [66]. Not only does this structure exhibit high stability, but it also effectively mitigates the blockage of electron transfer pathways caused by water molecules in the FeF₃∙0.5H₂O molecular structure. As a result, the material′s electrochemical performance is significantly enhanced.

To systematically compare the key characteristics of ReO₃-FeF₃, pyrochlore-type FeF₃·0.5H₂O, and HTB-FeF₃·0.33H₂O,

Table 3. Key characteristics of ReO₃-FeF₃, pyrochlore-type FeF₃·0.5H₂O, and HTB-FeF₃·0.33H₂O [11,62,63,64,65,66].

Property	ReO3-FeF3	Pyrochlore-FeF3·0.5H2O	HTB-FeF3·0.33H2O
Structure	Corner-sharing [FeF6] octahedra forming 3D cubic channels, Small pore size (~2.5 Å)	3D open framework with H2O in interstitial sites, Larger channels (~4.5 Å)	1D hexagonal tunnels with H2O in the center, Expanded interlayer spacing (~7.2 Å)
Lattice water content	None	0.5 H2O	0.33 H2O
Reaction mechanism	Dominant intercalation (~3.0 V)	Intercalation + partial conversion (~3.0 V and ~2.0 V)	Intercalation + reversible conversion (~3.0 V and ~2.0 V)
Theoretical capacity (mAh·g–1)	~237 (intercalation)	~300 (mixed)	~600 (mixed)
Advantages	High working voltage (~3.0 V), Low polarization	High structural stability, Enhanced Li+ diffusion due to H2O	High capacity, Good rate capability (~50% retention at 10C)
Disadvantages	Metastable (transforms to hexagonal phase), Low capacity	Structural collapse upon dehydration, Moderate capacity	Sensitive to H2O content, Capacity fading over long cycles
Modification strategies	Doping (Ti4+, Co2+)for stabilization, Nanostructuring	Carbon coating for conductivity, Interface engineering	Graphene composition, Precise H2O control

provides a multi-dimensional analysis of structural features, electrochemical behavior, and modification strategies. Due to differences in crystal configurations and water molecule regulation mechanisms, these three materials show significant variations in lithium-ion storage performance, with specific parameters compared in Table 3 as follows:

3.3. Synthesis–Structure–Performance Correlations

Electrochemical performance of iron fluorides is intrinsically linked to their synthesis methods and resultant crystal structures.

Table 4. Typical Products and Capacities of Iron Fluorides Prepared via Different Synthesis Routes.

Synthesis Method	Typical Product	Capacity (mAh·g−1)
Solid-State (Ball-milling/Mechanochemistry)	Anhydrous ReO3-FeF3 [62]	237
	FeF3/C composites [10]	450–712
Liquid-Phase (Hydrothermal/Solvothermal)	HTB-FeF3·0.33H2O [11]	600
	Pyrochlore-FeF3·0.5H2O [6]	300
Gas-Phase Fluorination (F2/HF treatment)	High-purity FeF3 [14]	200–400
	FeOXFY oxyfluorides [14]	500–600

constructs a three-dimensional correlation diagram of synthesis–structure–performance for iron-based fluorides. In terms of synthesis routes, solid-phase methods (such as ball milling and mechanochemical methods) can prepare anhydrous phases with high crystallinity (e.g., ReO₃-type FeF₃), but morphology control is limited [10]; liquid-phase methods (hydrothermal/solvothermal) can synthesize hydrated crystalline phases (e.g., HTB-type FeF₃·0.33H₂O) with adjustable nanostructures (nanowires, hollow spheres, etc.) [13,67]; gas-phase fluorination can obtain high-purity FeF₃ or FeOₓFᵧ oxyfluorides, but has strict requirements on process conditions [14]. Structural analysis shows that hydrated crystalline phases (pyrochlore/HTB-type) have expanded Li⁺ transport channels, but there is a problem of structural instability due to dehydration [6,12]; open framework structures (HTB and pyrochlore) are conducive to ion diffusion [11,66], while ReO₃-type FeF₃, although having a higher operating voltage, is limited by lower capacity [62]. Electrochemical performance tests show that the specific capacity of the materials ranges from 237 mAh/g for ReO₃-FeF₃ (intercalation reaction) to 712 mAh/g for nano FeF₃/C (complete conversion reaction) [10]. Modification strategies such as carbon coating and element doping (e.g., Co/O) can effectively inhibit volume expansion and achieve excellent cycling stability (e.g., FeF₃@C can stably cycle for >1000 times) [68].

In summary, the diverse polymorphic structures of iron-based fluorides offer rich possibilities for their application in LIBs. By controlling crystal structures, optimizing synthesis methods, and compositing with conductive matrices, researchers can further enhance the electrochemical performance of these materials, laying the foundation for their use in high-energy-density and high-power-density batteries. In the future, with a deeper understanding of the structure-property relationships of iron-based fluorides, these materials are expected to play an increasingly important role in next-generation energy storage technologies.

4. Key Challenges and Issues of Iron Fluoride Cathode Materials

To realize the practical application of iron fluoride, four major dilemmas must be addressed, as shown in Figure 5: (1) severe polarization caused by insulating properties due to strong ionic bonds; (2) electrode structure damage from dramatic volume expansion during conversion reactions; (3) poor reversibility resulting from difficult LiF decomposition and Fe nanoparticle agglomeration; (4) accelerated capacity decay due to electrode/electrolyte interfacial side reactions. These problems form a vicious cycle that limits performance [15,69,70].

4.1. Poor Intrinsic Conductivity

The primary challenge of iron fluoride cathode materials lies in their inherent insulating nature, with electronic conductivity below 10⁻¹⁰ S/cm. This extremely low conductivity leads to multiple adverse effects during electrochemical processes. The sluggish reaction kinetics result in significant polarization, where the discharge capacity at 1C rate is typically only ~30% of that at 0.1C. A pronounced voltage hysteresis is observed, with highly asymmetric charge/discharge profiles, reducing energy conversion efficiency to below 80% [13]. More critically, the lack of an effective conductive network prevents a large portion of active material from participating in electrochemical reactions, limiting practical capacity to only 30–50% of the theoretical value. Cycling stability is also severely impacted, with unmodified FeF₃ typically exhibiting capacity retention below 50% after 50 cycles, significantly hindering its practical application [72].

4.2. Severe Volume Effects During Cycling

Another major drawback of iron fluoride is its dramatic volume variation during electrochemical cycling. Under full conversion reaction conditions, the material undergoes volume expansion of up to 200%, generating substantial mechanical stress at the microscale. This stress induces microcracks within active particles, which propagate to the surface, eventually causing physical detachment from the conductive network. Experimental observations reveal that this structural degradation is cumulative, leading to over 40% capacity loss after 100 cycles [73]. Additionally, repeated volume fluctuations compromise electrode integrity, accelerating electrolyte penetration and side reactions, further deteriorating cycling performance. This volume effect, coupled with conductivity issues, collectively undermines long-term stability.

4.3. Poor Reversibility of Conversion Reactions

Iron fluoride suffers from severe reversibility issues in multi-electron conversion reactions during deep cycling. The reaction product LiF exhibits extremely high chemical stability and insulating properties (bandgap > 10 eV), making it difficult to fully decompose back into the original phase during charging. Meanwhile, the generated Fe nanoparticles tend to agglomerate and oxidize, leading to irreversible loss of active sites. Particularly in the low-potential region (<1.5 V vs. Li⁺/Li), side reactions reduce Coulombic efficiency to below 90%. These irreversible changes result in rapid capacity fading, with conversion reaction capacity often decaying by over 50% within 20 cycles. Furthermore, the instability of intermediate phases (e.g., LiFeF₃) exacerbates capacity degradation [74,75].

4.4. Complex Interfacial Side Reactions

The iron fluoride/electrolyte interface is plagued by intricate side reactions that severely impact performance. Highly reactive Fe³⁺/Fe²⁺ species are prone to dissolution and migration in conventional electrolytes, causing continuous active material loss. The unstable cathode-electrolyte interphase (CEI) undergoes repeated breakdown and reformation, depleting limited lithium reserves. Moreover, polyfluorinated iron intermediates (e.g., Li_xFeF₃) readily undergo disproportionation, forming electrochemically inactive phases. These interfacial issues lead to initial Coulombic efficiency typically below 80%, with capacity decay rates increasing by 3–5 times at elevated temperatures (60 °C). Notably, interfacial side reactions synergize with volume effects and conductivity limitations, creating a vicious cycle that accelerates performance degradation [76].

These challenges fundamentally stem from conflicts between iron fluoride chemical bonding characteristics and reaction mechanisms. Fortunately, recent strategies in nanoengineering, crystal structure control, and interface modification have demonstrated potential to synergistically address these issues, providing a theoretical basis for discussing modification methods.

5. Modification of Iron Fluoride Cathode Materials

Recent studies have revealed several inherent limitations of iron fluoride that hinder its practical application in battery systems. The material demonstrates high chemical reactivity in ambient conditions, readily absorbing moisture to form crystalline hydrates while generating free fluoride ions (F⁻) through hydrolysis reactions [77]. Furthermore, its strong Fe-F ionic bonding and wide bandgap result in poor electronic conductivity, significantly limiting capacity enhancement and making it challenging to meet the demanding requirements of high-capacity batteries. Most critically, iron fluoride undergoes irreversible phase transformation into electrochemically distinct LiF and metallic Fe phases during cycling, accompanied by substantial volume variations that mechanically degrade electrode integrity and lead to rapid performance deterioration. To address these challenges, researchers have developed comprehensive modification strategies including nanostructure engineering to optimize particle morphology and shorten ion transport paths, open-framework designs to create buffering spaces and enhance electrolyte penetration, anion/cation doping to improve intrinsic conductivity through lattice optimization, and surface/interface modification to stabilize the electrode–electrolyte interface. These synergistic approaches collectively enhance iron fluoride conductive network, structural stability and interfacial properties, providing promising solutions for overcoming its fundamental limitations in advanced battery technologies [78,79].

5.1. Nanoengineering

The advent and evolution of nanotechnology have pioneered novel pathways and conceptual frameworks for battery material design at the nanoscale, offering transformative solutions to longstanding challenges in conventional battery materials. First, nanomaterials exhibit pronounced size effects that dramatically reduce lithium-ion and electron transport pathways within active phases, thereby substantially enhancing both electronic and ionic conductivity. Second, the surface effects characteristic of nanomaterials enable significant expansion of the interfacial contact area between active materials and electrolytes. This leads to multiplicative increases in electrochemically active sites, markedly improving the reversibility of electrode reactions.

Li et al. successfully synthesized FeF₃·3H₂O nanowires through a dislocation-driven growth method in an ethanol solution with low supersaturation, using HF acid as the fluorine source [13]. Subsequent thermal treatment at 350 °C under Ar atmosphere effectively removed crystalline water from the as-synthesized nanowires, yielding porous r-FeF₃ nanowires, as shown in (Figure 6a). When employed as cathode material in LIBs, these r-FeF₃ nanowires demonstrated remarkable electrochemical performance. The material delivered an initial discharge capacity of 543 mAh·g⁻¹ and maintained 223 mAh·g⁻¹ after 50 cycles at 50 mA·g⁻¹ under room temperature, as shown in (Figure 6b). The outstanding performance originates from the unique structural advantages of the r-FeF₃ nanowires: the large surface area provides abundant active sites for electrochemical reactions, facilitating lithium ion intercalation/deintercalation; the robust architecture effectively accommodates structural strain during charge/discharge processes, minimizing material degradation; the continuous conductive network formed by iron nanodomains significantly enhances electronic conductivity, promoting rapid electrode kinetics.

Lian et al. designed a hierarchically mesoporous FeF₃/reduced graphene oxide (rGO) hybrid material [67], where hollow FeF₃ nanospheres serve as the main capacity contributor while two-dimensional rGO nanosheets act as a conductive matrix to enhance electronic conductivity and buffer volume expansion. This unique FeF₃/rGO hybrid was rationally synthesized via a non-aqueous in situ precipitation method, endowing it with multiple advantages including a large specific surface area, abundant active sites, fast lithium-ion transport channels, effective mitigation of volume expansion during cycling, and accelerated electrochemical reaction kinetics. Experimental results demonstrated that the FeF₃/rGO hybrid electrode exhibited exceptional electrochemical performance, delivering a high initial discharge capacity of 553.9 mAh·g⁻¹ at 0.5C and maintaining a stable capacity of 378 mAh·g⁻¹ after 100 cycles, indicating excellent cycling stability. Even at a higher rate of 2C, the hybrid electrode still delivered a respectable capacity of 168 mAh·g⁻¹, demonstrating good rate capability. The superior performance can be attributed to the synergistic effects between the hollow FeF₃ nanospheres and conductive rGO network, which not only facilitates electron/ion transport but also effectively accommodates structural strain during charge/discharge processes.

Shi et al. successfully fabricated a novel FeF₃·0.33H₂O@hollow acetylene black (FeF₃·0.33H₂O@HAB) composite material through an innovative synthesis approach [80]. Comprehensive characterization using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) confirmed the uniform distribution of FeF₃·0.33H₂O nanoparticles on the surface of hollow acetylene black nanospheres, forming a unique core–shell architecture. When evaluated as a cathode material for LIBs, the composite exhibited exceptional electrochemical performance with an initial discharge capacity of 224.4 mAh·g⁻¹ at 0.1C rate. Remarkably, it maintained a reversible capacity of 162.3 mAh·g⁻¹ after 1000 cycles, demonstrating outstanding cycling stability with a capacity retention of 72.3% and an ultralow capacity decay rate of only 0.027% per cycle. This superior performance originates from the material’s distinctive structural advantages: the hollow acetylene black matrix establishes an efficient conductive network that significantly enhances electron transport while effectively buffering volume variations during charge/discharge processes, and the strong interfacial integration between components ensures structural stability and sustained electrochemical activity.

Li et al. successfully synthesized a three-dimensional (3D) hybrid nanocomposite by incorporating Fe₂F₅·H₂O (FF) into N-doped porous carbon (NPC) [12]. This composite material combines the high reactivity of nanosized iron fluoride particles with the superior conductivity of the NPC matrix, not only providing an enlarged surface area for electrochemical reactions but also significantly shortening Li⁺ diffusion pathways, thereby enhancing the overall conductivity of the material. Experimental results demonstrate that the composite maintained a specific capacity of mAh·g⁻¹ after 200 cycles at 2C rate, exhibiting exceptional long-term cycling stability. Even under a high current rate of 5C, it delivered a remarkable specific capacity of 115 mAh·g⁻¹, showcasing outstanding rate capability. The superior performance can be attributed to the synergistic effects between the highly active Fe₂F₅·H₂O nanoparticles and the conductive NPC framework, which effectively facilitates charge transfer while accommodating volume changes during cycling.

Kang et al. developed a novel CNT-FeF₃ composite material (designated as FNCB) by growing FeF₃ on chemically etched carbon nanotubes [5]. The etching treatment intentionally created surface defects on the CNTs, resulting in a distinctive nanoflower-like morphology that provided favorable nucleation sites for FeF₃ deposition (Figure 7a,b). This hierarchical nanoarchitecture of the CNT-FeF₃ composite significantly improves interfacial contact between active materials and conductive agents while establishing efficient transport pathways for both electrons and ions. The FNCB composite demonstrated remarkable electrochemical performance, delivering a specific capacity of approximately 210 mAh·g^–1 at a current density of 20 mA·g^–1 (Figure 7c). Furthermore, it exhibited excellent high-rate capability, maintaining a substantial capacity of about 150 mAh·g^–1 even at an elevated current density of 500 mA·g^–1 (Figure 7d). These superior properties stem from the synergistic combination of the conductive CNT network and the well-dispersed FeF₃ nanoparticles, which collectively enhance charge transfer kinetics while accommodating volume changes during cycling.

Guan et al. reported a universally applicable one-step fusion synthesis method to successfully construct a heterostacked nanoarchitecture of iron fluoride nanocomposite (FeF₃@C-Asphalt) [16]. In this unique structure, ultrafine FeF₃ nanoparticles are uniformly encapsulated within a highly conductive carbon framework (Figure 8a). Through electrochemical kinetic calculations and multiphysics simulations, the nanocomposite was found to consist of ultrafine nanoparticles confined in a carbon matrix, exhibiting a high tap density of 1.8 g/cm³. This characteristic not only significantly enhances conductivity and improves charge transport pathways, enabling rapid electron transfer and ion migration, but also reduces internal electrode stress and effectively mitigates volume expansion. The optimized FeF₃@C-Asphalt cathode demonstrated exceptional electrochemical performance, delivering a high capacity of 517 mAh·g⁻¹ at 5 A·g⁻¹ (10C) while maintaining 87.5% capacity retention after 1000 cycles (Figure 8b). This outstanding performance originates from the synergistic effects between the nanosized FeF₃ particles and conductive carbon network: (1) the carbon coating provides continuous electron transport pathways; (2) the nanoconfinement effect buffers volume changes during cycling; (3) the dense packing of active materials ensures high volumetric energy density.

5.2. Open-Framework Strategy

The development of open-framework strategies has revolutionized electrode material design by providing innovative solutions to critical performance limitations through controllable porous architectures. First, the hierarchical pore structure characteristic of open-framework materials establishes continuous ion transport networks, reducing lithium-ion diffusion distances to the nanoscale and significantly enhancing intrinsic ionic conductivity. Second, the three-dimensional interconnected conductive framework not only provides efficient electron transport pathways but also exposes abundant active sites through its exceptionally high specific surface area (typically >500 m²/g), dramatically improving reaction kinetics. Third, the adaptive buffer spaces within the open-framework structure can effectively accommodate volume changes during charge/discharge cycles (strain tolerance >30%), fundamentally addressing the structural instability issues of conventional electrode materials.

Li et al. employed an open-framework strategy to successfully synthesize various mineral phases, including hexagonal tungsten bronze (HTB), tetragonal tungsten bronze (TTB), and pyrochlore-type iron fluorides [12]. These polymorphs were formed using ionic liquid (IL) reaction media as both structure-directing agents and soft templates. Notably, the framework construction process typically involves the incorporation of guest molecules (e.g., water) or ions (e.g., potassium ions) into structural channels. These species pre-occupy potential lithium intercalation sites, thereby limiting additional Li⁺ storage capacity. Moreover, the release of channel-confined water molecules into the electrolyte may degrade battery performance. Consequently, the removal of channel water is essential for electrode materials, though this process may induce structural distortion or even amorphization of the original phase. For instance, pyrochlore-type FeF₃·0.5H₂O undergoes topological densification upon dehydration, transforming into a short-range-ordered amorphous FeF₃ phase. Similar improvements were observed in hexacyanometallate frameworks with favorable symmetry changes, where the removal of interstitial water molecules significantly mitigated voltage polarization and enhanced rate capability.

Capitalizing on the unique structural characteristics of the HTB phase featuring straight, open channels that theoretically enable complete removal of interstitial water molecules, they achieved a breakthrough by developing an innovative ionothermal fluorination approach. Using microphase-separated ionic liquids as both reaction medium and structural template, they successfully synthesized dehydrated bronze-type iron fluoride with perfect HTB structure (Figure 9a). The resulting material exhibits exceptional crystallinity and pristine, uncoated surfaces, which proved critical for complete water molecule elimination and optimal electrochemical performance. When tested as a conversion-type cathode in LIBs, this dehydrated HTB-phase iron fluoride demonstrated outstanding performance, delivering reversible capacities of 200–450 mAh·g⁻¹ for over 100 cycles—representing an approximately 100 mAh·g⁻¹ improvement compared to conventional hydrated HTB phases (Figure 9b,c)—while maintaining excellent cycling stability. This significant advancement stems from the precisely controlled ionothermal conditions that preserve structural integrity during dehydration, combined with the unique templating effect of microphase-separated ionic liquids that facilitate the formation of perfectly ordered, water-free channels without surface contamination.

The pyrochlore-structured iron(III) fluoride (Pyr-FeF₃) features large hexagonal channels formed by corner-sharing FeF₆ octahedra. Compared to rhenium trioxide-type iron(III) fluoride (r-FeF₃) with narrower lithium diffusion pathways, this structural characteristic potentially enhances lithium-ion diffusion, thereby improving rate capability and reducing voltage hysteresis. While structurally related compounds—tetragonal iron(III) fluoride dihydrate (FeF₃(H₂O)₂·H₂O) and hexagonal tungsten bronze-type iron fluoride (FeF₃·0.33H₂O)—also contain internal channels, these are limited to one-dimensional pathways that may become obstructed during prolonged cycling-induced amorphization (loss of long-range order). In contrast, Pyr-FeF₃ maintains interconnected three-dimensional channels along the (110) and equivalent directions that likely preserve some connectivity even after amorphization, thereby sustaining lithium diffusion pathways. Similarly, Baumgärtner et al. developed a cost-effective, highly scalable synthesis method with 100% atom economy for pyrochlore-type iron hydroxyfluoride (Pyr-IHF) [6]. This approach effectively transforms the 1D channels in tetragonal FeF₃(H₂O)₂·H₂O precursor into 3D lithium conduction pathways in cubic Pyr-IHF product, creating a framework structure that enables lithium diffusion through 3D hexagonal channels formed by corner-sharing FeF_6−x(OH)_x octahedra (Figure 10a). Subsequent thermal treatment selectively produced Pyr-IHF cathodes with varying water content. The removal of crystalline water from 3D channels significantly enhanced battery capacity at medium-to-high current densities (Figure 10b), demonstrating the detrimental effect of channel water on capacity and providing the first direct evidence for lithium diffusion along pyrochlore’s three-dimensional channels.

5.3. Anionic/Cationic Doping

The introduction of oxygen into metal fluorides (MFs) to form metal oxyfluorides effectively combines the advantages of both fluorides and oxides, offering promising solutions to achieve ideal battery characteristics including high capacity, enhanced conductivity, and improved cycling stability, where the substitution of anions significantly influences the crystal lattice structure and consequently modifies the physicochemical properties of MFs. For instance, the FeOF metal oxyfluoride exhibited unique properties with an exceptionally high theoretical capacity of 885 mAh·g⁻¹ [81], far surpassing conventional FeF₂ and FeF₃ cathode materials, as it not only retains the high reactivity and elevated output voltage characteristics endowed by fluorine atoms but also benefits from the oxide’s positive effects on capacity enhancement and cycling stability. Liu et al. demonstrated an iron oxyfluoride–graphene (FeOF-G) nanocomposite synthesized via solution and hydrothermal methods [82], comprising graphene sheets supporting nanostructured FeOF particles, where the composite architecture enables (1) effective immobilization of FeOF nanoparticles along with discharge products (Fe nanoparticles and LiF) between graphene layers to prevent their migration and dissolution into the electrolyte, thereby ensuring reversible (de)lithiation processes, and (2) establishment of interparticle conductivity. This FeOF-G nanocomposite exhibited a distinct yet fully reversible (de)lithiation mechanism that allows recovery of its original chemical composition and morphological structure after delithiation, involving during lithiation an extended intercalation process (0.8 e⁻ transfer compared to 0.6 e⁻ for pure FeOF) followed by conversion first to metallic Fe and LiF (first stage) and then to Fe and Li₂O (second stage), while during subsequent delithiation, Fe, LiF and Li₂O can progressively and reversibly transform back to FeOF-G—a feature unattainable with pure FeOF. Through this unique (de)lithiation process, the FeOF-G achieved remarkable electrochemical performance including a reversible capacity of 621 mAh·g⁻¹, specific energy of 1124 Wh·kg⁻¹ (corresponding to 2.1 valence changes/storage of 2.1 Li⁺ per unit), and capacity retention of 80% after 100 cycles at 0.1C, representing a threefold improvement over conventional intercalation-type LiCoO₂ (145 mAh·g⁻¹ and 551 Wh·kg⁻¹).

Liu et al. developed an innovative three-dimensional sandwich-structured composite through coprecipitation followed by calcination, consisting of nickel/cobalt dual-doped FeF₃·0.33H₂O nanoparticles (NC-FF) encapsulated within reduced graphene oxide (rGO) sheets [7]. In this architecture, the flower-like nickel/cobalt co-doped FeF₃·0.33H₂O nanoparticles are firmly anchored between highly conductive rGO layers through robust M-O-C bonding interactions, creating both a stable electron/ion transport network and an effective buffer layer to accommodate substantial volume changes during cycling. The nickel doping contributes to enhanced capacity by expanding the unit cell volume, while cobalt doping improves rate capability and cycling stability by accelerating lithium-ion migration. Benefiting from the synergistic effects of dual metal doping and rGO encapsulation, the NC-FF composite demonstrates exceptional electrochemical performance, delivering a high-rate capacity of 200.1 mAh·g⁻¹ at 5C and maintaining excellent cycling stability with a retained capacity of 177.8 mAh·g⁻¹ after 400 cycles—a performance that underscores the effectiveness of this dual-doping strategy combined with conductive carbon confinement for developing advanced iron fluoride-based cathode materials.

Fan et al. employed a solvothermal method to synthesize Fe_0.9Co_0.1OF through a Co and O co-substitution strategy, successfully optimizing the cycling stability and reaction kinetics of FeF₃ [14]. During charge/discharge processes, Fe_0.9Co_0.1OF underwent lithium intercalation followed by a two-step conversion reaction, both demonstrating high reversibility. The co-substitution of Co and O in FeF₃ significantly reduced the voltage hysteresis to 0.27 V while effectively decreasing both particle size and abundance of metallic Fe(Co), thereby suppressing irreversible conversion reactions. The Co doping specifically reduced nucleation size and formation energy of the metallic phase, diminishing metal particle content and size, which lowered the thermodynamic potential of conversion reactions and inhibits their progression. Simultaneously, the formed non-stoichiometric rock-salt phase substantially enhanced the reversibility of conversion reactions. The optimized Fe_0.9Co_0.1OF exhibited outstanding performance, achieving an energy density of 1000 Wh·kg⁻¹ (corresponding to a specific capacity of 420 mAh·g⁻¹) at 70 mA·g⁻¹ current density while maintaining stable cycling over 330 cycles—a remarkable improvement that demonstrates the effectiveness of this dual-element substitution approach for developing high-performance conversion-type cathode materials with both high energy density and long-term stability.

5.4. Surface/Interface Modification

Surface modification plays a multifaceted role in enhancing the performance of iron fluoride electrodes. Firstly, it stabilizes the electrode-electrolyte interface, effectively suppressing phase transitions and associated performance fluctuations. Secondly, it improves both ionic and electronic conductivity, facilitating efficient electrochemical reactions. Thirdly, it reduces electrolyte acidity, mitigating electrode corrosion in acidic environments. Fourthly, it helps accommodate substantial volume changes during charge/discharge cycles, maintaining structural integrity. Through these mechanisms, surface modification ultimately enhances reversible capacity and cycling stability [59,71].

Kim et al. developed a graphitic carbon-coated FeF₃ nanoparticle composite via a simple polymerization method using FeCl₃ as the iron precursor, citric acid (C₆H₈O₇) as both carbon source and chelating agent, and ethylene glycol (EG) as crosslinker [8] (Figure 11). The in situ generated Fe⁰ catalyzed graphitic carbon formation before converting to FeF₃. The composite demonstrated superior performance compared to bare FeF₃, delivering an initial discharge capacity of ~188 mAh·g⁻¹ at 0.1C (2.0–4.5V) with a low-capacity decay rate of 0.24% over 50 cycles. After a gradual activation process in a wider voltage window (1.5–4.5V), the discharge capacity increased to 421 mAh·g⁻¹, building upon the initial ~374 mAh·g⁻¹ capacity observed in the first cycle. The composite also showed significantly improved rate capability compared to pure FeF₃. These enhanced lithium storage properties primarily stem from the controlled FeF₃ nanoparticle size and the conductive graphitic carbon layer that continuously coats the FeF₃ surface throughout cycling.

Shi et al. successfully fabricated a three-dimensional porous nitrogen-doped carbon matrix with in situ anchored FeF₃ nanocavities coated by graphitic carbon (FeF₃/GC) through an innovative iron–carbon precursor self-bubbling method followed by vapor-phase diffusion [68]. The Kirkendall diffusion effect induced during vapor-phase diffusion caused metastable Fe₃C particles to undergo particle fragmentation and diffusion-recombination processes, ultimately forming FeF₃ nanoholes in intimate contact with graphitic carbon layers. This interpenetrating hybrid nanostructure, featuring FeF₃ nanocavities, provided abundant active sites for lithium storage while effectively shortening electron/ion diffusion pathways and enhancing reaction kinetics during multi-electron processes. The FeF₃/GC composite demonstrated extraordinary electrochemical performance, delivering reversible specific capacities of 564.5 mAh·g⁻¹ after 400 cycles and maintaining 504.2 mAh·g⁻¹ after 1200 cycles at a high current density of 1000 mA·g⁻¹, with an ultralow capacity decay rate of merely 0.01% per cycle. This remarkable cycling stability, combined with high-capacity retention, highlights the effectiveness of the unique nanocavity structure and conductive carbon network in addressing the intrinsic challenges of iron fluoride electrodes.

In addition to carbon-based materials, various substances have also been employed for surface modification of electrode materials, including metal oxides (TiO₂, V₂O₅, Al₂O₃, CeO₂, ZrO₂, SiO₂, ZnO), phosphates (AlPO₄), and metal fluorides (AlF₃ and Li₃FeF₆). Zhang et al. successfully synthesized spherical nano-TiO₂-coated FeF₃·0.33H₂O composites for lithium-ion batteries through a combined solvothermal and sol–gel coating approach [72]. The TiO₂-modified FeF₃·0.33H₂O demonstrated exceptional electrochemical properties: a high specific capacity (initial discharge capacity of 654 mAh·g⁻¹ at 0.1C rate), long cycle life (75.3% capacity retention after 200 cycles at 0.2C), excellent rate capability, and satisfactory initial Coulombic efficiency. This remarkable performance enhancement primarily stems from the synergistic effects between the spherical FeF₃·0.33H₂O core and the nano-TiO₂ coating layer, along with the unique advantages of the spherical morphology, which collectively enhance diffusion kinetics and preserve structural stability during cycling. The conformal TiO₂ coating not only provided physical protection against electrolyte decomposition but also facilitated lithium-ion transport while suppressing undesirable side reactions, demonstrating the effectiveness of metal oxide coatings as an alternative to conventional carbon-based surface modifications for iron fluoride cathode materials.

Through rational electrolyte design and optimization, the interfacial stability of iron fluoride-based electrodes can be significantly improved, thereby enhancing the overall electrochemical performance of batteries. Such electrolyte optimization improves the electrode/electrolyte interface through the following approaches: forming stable SEI/CEI layers to inhibit side reactions and the dissolution of metal ions, regulating the interfacial ion transport kinetics to reduce charge transfer resistance, and buffering the volume changes in electrode materials during charge–discharge processes.

Yushin et al. [83] investigated the influence of different liquid electrolytes (such as combinations of LiPF₆, LiTFSI, and LiFSI salts in FEC-EMC or DME solutions) on the performance of FeF₂/CNT composites. They found that DME-based electrolytes exhibit excellent performance rate, while LiTFSI salt in FEC-EMC performs the best due to its ability to form a stable CEI film. Xiao et al. [84] achieved high theoretical capacity and long cycling stability (capacity retention >90% after 50 cycles) for monodispersed FeF₂ nanorods using an ionic liquid electrolyte (1 M LiFSI/Pyr_1,3FSI). This is attributed to the stable SEI film formed and its ability to inhibit particle aggregation. Chen et al. [85] introduced tris(pentafluorophenyl)borane (TPFPB) additive into ether-based electrolytes. By forming [TPFPB−F]⁻ complexes to promote LiF decomposition, a solid–liquid “F transport channel” was constructed, which significantly improved the reversible conversion reaction efficiency of iron oxyfluoride cathodes. Yushin et al. [86] replaced organic electrolytes with solid polymer electrolytes (SPE), achieving long cycling stability (>300 cycles) and high capacity (>450 mAh g⁻¹) for FeF₂ cathodes at 50 °C. The high elasticity and electrochemical stability of SPE form a uniform CEI film with high mechanical strength, effectively inhibiting the dissolution and degradation of cathode materials. Baumgärtner et al. [6] demonstrated that Pyr_1,4TFSI ionic liquid electrolyte can significantly reduce side reactions between pyrochlore-type hydroxyiron fluoride (Pyr-HIF) cathode and Li₆PS₅Cl solid electrolyte, but interfacial reactions still result in capacity contributed by sulfur redox. Hu et al. [15] confirmed that ceramic solid electrolytes (e.g., Li₃FeF₆) can stabilize the interface of FeF₃ at high temperatures, providing a new idea for solving interface compatibility issues in solid-state batteries.

Sun et al. [87] first designed a composite system of mesoporous FeF₂ (meso-FeF₂) and in situ polymerized PEGMA-LiTFSI-SPE, achieving a high energy density of 1023 Wh kg⁻¹ and a capacity retention of >460 mAh g⁻¹ after 100 cycles at 60 °C. The uniform SEI layer formed by the SPE can effectively inhibit Fe²⁺ dissolution. It was found that the pore size (7.5 nm) and specific surface area (53.3 m² g⁻¹) of meso-FeF₂ can buffer volume changes, while in situ polymerization ensures tight contact between the electrolyte and the electrode.

5.5. Comparative Performance of Modified Iron Fluoride Cathodes

The preceding sections (Section 5.1, Section 5.2, Section 5.3 and Section 5.4) have systematically examined nanoengineering, open-framework design, doping strategies, and surface modifications for iron fluoride cathodes. To quantitatively evaluate the efficacy of these approaches,

Table 5. Summary of selected works on the cycling performance of iron fluorides as cathode materials for lithium-ion batteries.

Samples	Voltage Range /V	Current Density /mAg–1	Capacity/mAh g−1	Cycle Number
FeF2.2(OH)0.8·0.33H2O	1−4.5	20	170 [88]	50
FE-FeF2	1–4.5	50	177 [93]	30
FeF3·0.33H2O/C + G	1.8–4.5	47.4	193.1 [79]	50
Cr-doped Fe2F5·H2O	1.0–4.0	20	171 [91]	100
FeF3·0.5H2O	1.7–4.5	11	145 [94]	100
FeF3·0.5H2O	1.7–4.5	24	135 [12]	300
FeF3/C/RGO	1.0–4.0	100	220 [89]	200
FeF3/C	1.5–4.5	7.12	250 [95]	100
FeF2-CMK-3	1.5–4.5	500	529 [96]	100
FeF3/rGO	1.7–4.5	1000	146 [97]	50
N-doped FeF3/C	2.0–4.5	5000	95 [98]	250
FeF3-carbon nanofiber	1.0–4.0	100	550 [90]	400
FeF2@CNT nanorods	1.3–4.3	50	263 [99]	50
FeF3/MoS2	2.0–4.5	23.7	169.6 [100]	30
Ni-doped FeF3·0.33H2O	1.5–4.5	200	264 [92]	100
FeF3·0.33H2O	2.0–4.5	200	167 [101]	100
Lithiated FeF3/C	1.0–4.5	25	400 [102]	30

consolidates the electrochemical performance metrics of representative modified iron fluoride systems reported in recent literature. For instance, hydrated phases such as FeF_2.2(OH)_0.8·0.33H₂O and FeF₃·0.33H₂O/C + G exhibit moderate capacities (170–193 mAh g⁻¹) at low current densities (20–50 mA/g), consistent with their known limitations in electronic conductivity and phase stability [79,88]. In contrast, advanced carbon composites (e.g., FeF₃/C/RGO, FeF₂-CMK-3) demonstrate significantly enhanced performance, delivering capacities exceeding 220 mAh g⁻¹ over 100–200 cycles, attributed to improved charge transport and reduced particle agglomeration [79,89]. Notably, nanostructured systems like FeF₃-carbon nanofiber achieve exceptional capacities (~550 mAh g⁻¹) with prolonged cycling (400 cycles), underscoring the critical role of conductive matrices in mitigating capacity fade [90]. Doping strategies (e.g., Cr-doped Fe₂F₅·H₂O, Ni-doped FeF₃·0.33H₂O) further enhance reversible lithium storage, albeit with trade-offs in rate capability [91,92]. Three key trends can be derived from these comparative analyses: (i) Carbon-based composites (e.g., FeF₃/RGO) consistently exhibit excellent cycling stability (>200 cycles) due to enhanced electron transport capability (see Section 5.1); (ii) Hydrated phases (e.g., FeF₃·0.33H₂O) show a 30–50% reduction in voltage hysteresis compared to anhydrous phases; (iii) Transition metal doping (e.g., Cr/Ni) improves capacity retention by more than 40% through lattice stabilization. Notably, the highest achieved capacity (550 mAh g⁻¹ at a current density of 100 mA g⁻¹), which combines conductive carbon nanofiber networks and morphological regulation, approaches 77% of the theoretical maximum (712 mAh g⁻¹). These empirical results highlight the necessity of adopting synergistic modification strategies to overcome the inherent limitations outlined in Section 4.

The aforementioned modification strategies provide crucial solutions to address the inherent challenges of iron fluorides—low conductivity and substantial volume variation—significantly enhancing their electrochemical performance. The nanosizing approach dramatically improves kinetic properties by reducing particle size to the nanoscale, which shortens lithium-ion diffusion paths and decreases ionic transport resistance. Simultaneously, the smaller particle dimensions help mitigate stress induced by volume changes during cycling, partially preventing material pulverization. Elemental doping modifies the crystal structure and electron cloud distribution of iron fluorides through the introduction of foreign metal or nonmetal species with varying electronegativities and ionic radii, thereby optimizing intrinsic conductivity. Carbon coating strategies establish continuous electron-conducting networks that markedly enhance electronic conductivity, while the flexible carbon layers accommodate volume expansion and maintain electrode structural integrity. These multidimensional modification approaches not only offer diverse technical pathways to overcome the material’s inherent limitations but also establish a solid theoretical and practical foundation for developing novel optimization strategies and further performance enhancement.

Beyond intrinsic material properties, the electrochemical performance of iron fluoride cathodes is critically influenced by formation protocols. Recent studies highlight that gradual activation cycles (e.g., stepwise voltage window expansion from 2.0–4.5 V to 1.5–4.5 V) significantly enhance reversible capacity by stabilizing the electrode–electrolyte interface and mitigating irreversible phase transitions [8,10]. Temperature also plays a dual role: While elevated temperatures (~70 °C) improve Li⁺ diffusion kinetics, they may accelerate parasitic reactions (e.g., Fe dissolution) and CEI degradation [15,76]. Conversely, low temperatures exacerbate polarization due to sluggish ionic transport. Additionally, external pressure (e.g., 10–20 MPa in solid-state configurations) has been shown to suppress particle cracking induced by volume expansion, synergizing with nanostructure design [15]. Future efforts should prioritize protocol standardization (e.g., voltage/current thresholds), temperature-adaptive electrolytes, and pressure-engineered cell architectures to bridge the gap between laboratory research and industrial deployment.

6. Summary and Prospects

The systematic modification strategies discussed in Section 3, Section 4 and Section 5 (nanoengineering, open-framework design, doping, and interface engineering) have significantly mitigated the intrinsic limitations of iron fluoride cathodes, including poor conductivity, volume expansion, and irreversible conversion reactions. However, for practical deployment, a holistic evaluation must extend beyond electrochemical performance to encompass manufacturing scalability, cost competitiveness, and environmental impact—dimensions where conventional cathode materials (e.g., LCO, NCM, LFP) have established benchmarks.

Table 6. Comparative Analysis of Cathode Materials: Commercial Viability, Manufacturing Feasibility, and Environmental Impact.

Material Type	Commercial Viability	Manufacturing Feasibility	Environmental Concerns
Iron Fluoride (FeF3)	Low cost Highest capacity (712 mAh·g−1 theoretical)	HF fluorination (high-risk) High energy input	Cobalt/nickel-free Hydrometallurgical recycling (HF handling) Medium carbon emissions
Lithium Cobalt Oxide (LCO)	High cost Medium capacity (274 mAh·g−1)	Solid-state reaction (mature) Medium energy input	Cobalt mining pollution Pyrometallurgical recycling (high energy) High carbon emissions
NCM811	Medium-high cost High capacity (220 mAh·g−1)	Co-precipitation (complex) High energy input	Nickel/cobalt ecological impact Pyrometallurgical challenges Very high carbon emissions
Lithium Iron Phosphate (LFP)	Low cost Low capacity (170 mAh·g−1)	Solid-phase synthesis (simple) Low energy input	Heavy-metal-free Easy acid-leach recycling Low carbon emissions

summarizes the comparative analysis of iron fluoride (FeF₃) cathode materials versus conventional materials across three critical dimensions: commercial viability, manufacturing feasibility, and environmental concerns. Iron fluoride (FeF₃) exhibits significant advantages and challenges as a next-generation cathode material for Li/Na-ion batteries. In terms of performance, its theoretical capacity reaches an impressive 712 mAh·g⁻¹, far exceeding that of conventional materials (LCO: 274 mAh·g⁻¹, NCM811: 220 mAh·g⁻¹, LFP: 170 mAh·g⁻¹). From a cost perspective, FeF₃ enjoys raw material cost advantages due to the abundant resources of iron (~$0.1/kg) and fluorine (derived from low-cost fluorite/CaF₂). Some optimized formulations, such as FeF₃@C-Asphalt nanocomposites, utilize low-cost petroleum asphalt (~$0.5/kg) to achieve excellent cycling performance.

However, its commercialization faces multiple challenges. In terms of manufacturing processes, traditional fluorination generates highly toxic HF, requiring corrosion-resistant equipment and high energy input, resulting in higher costs compared to the mature production technologies of conventional cathodes (solid-state synthesis for LCO, co-precipitation for NCM811, and solid-phase synthesis for LFP). For large-scale production, open-framework structures like dehydrated HTB-FeF₃ require ionothermal synthesis conditions at 300–400 °C, while modification methods such as graphene coating ($100–500/kg) or Co/Ni doping (cobalt at approximately $30/kg) further increase the costs. In the recycling stage, specialized hydrometallurgical technologies need to be developed to handle HF by-products, which is different from the pyrometallurgical recycling of LCO/NCM811 or the environmentally friendly acid leaching process of LFP.

To address key challenges existing in conversion-type iron fluoride materials such as poor intrinsic conductivity (<10⁻¹⁰ S/cm), significant volume changes during charge/discharge cycles (~200%), and low reversibility of conversion reactions, researchers have developed four main modification strategies: (1) nanoengineering designs (e.g., hollow nanospheres, graphene-wrapped nanowires) to shorten ion transport paths; (2) open-framework structural optimization to enhance lithium-ion diffusion channels; (3) anionic/cationic doping (e.g., Co/O co-doping) to regulate electronic structure; (4) surface/interface modifications (e.g., graphitic carbon coating, TiO₂ layers) to stabilize electrode-electrolyte interfaces. The synergistic application of these strategies has led to remarkable performance improvements, such as Fe_0.9Co_0.1OF achieving an energy density of 1000 Wh·kg⁻¹ [14], while graphitic carbon-coated samples maintained a capacity of 504.2 mAh·g⁻¹ after 1200 cycles [68].

Despite these significant advancements, iron fluoride cathodes still face three core challenges: (1) kinetic limitations—high energy barriers for LiF decomposition leading to charging difficulties, with practical capacities typically below 60% of theoretical values; (2) engineering hurdles—low tap density of nanomaterials (<2.0 g/cm³) and complex synthesis processes (e.g., ionothermal methods) hindering mass production; (3) interface instability—conventional carbonate electrolytes decomposing at high voltages, coupled with active material dissolution and repeated cracking of the CEI layer causing continuous capacity fading.

To propel iron-based fluoride cathodes toward practical implementation, three critical research directions must be prioritized. First, advanced material modification strategies should be further developed, particularly through high-entropy doping and nanostructural engineering, to simultaneously address the intrinsic limitations of poor conductivity and severe volume changes. Second, in-depth mechanism research using cutting-edge in situ/operando characterization techniques is essential to fully elucidate the complex phase transition behaviors and interfacial evolution during cycling, which will guide more rational material design. Third, system-level integration considerations must be pursued, including compatibility studies with emerging battery configurations such as solid-state electrolytes and lithium-metal-free anodes, as well as the development of sustainable large-scale synthesis and recycling processes. The synergistic advancement of these aspects will not only overcome existing challenges but also establish iron fluorides as a competitive candidate for next-generation high-energy-density batteries, potentially surpassing the performance ceilings of conventional intercalation-type cathode materials.

In summary, systematic structural design and performance optimization have led to major progress in iron fluoride cathode materials. However, achieving practical applications will require continued innovation in material preparation, interface stability, and system integration. With breakthroughs in key technologies, these materials are poised to make significant contributions to the development of next-generation high-energy-density batteries.