Unveiling the Performance Symphony of Iron Fluoride Cathodes in Advanced Energy Storage Devices

Abstract

Increasing the storage capacity of portable electronic storage devices is one example of how energy storage and conversion have recently emerged as key research subjects for addressing social and environmental concerns. Metal fluoride cathodes have recently received a lot of attention as potential components for high-performance lithium batteries. These cathodes have unique electrochemical properties that make them suitable for energy storage applications. The research looks into the electrochemical properties of numerous metal fluorides, including transition metal fluorides and rare earth metal fluorides. The study finishes by emphasising metal fluoride cathode applications in portable electronics, electric cars and grid energy storage.

Introduction

In recent years, the demand for high-performance lithium batteries has increased significantly due to their crucial role in various applications, including portable electronics, electric vehicles, and grid energy storage. To meet the ever-growing energy storage needs, researchers have been exploring new cathode materials with enhanced electrochemical performance. Amongst these materials, metal fluorides have emerged as promising candidates, offering unique characteristics that make them attractive for advanced lithium batteries. The purpose of this research is to shed light on the electrochemical capabilities of metal fluoride cathodes and discuss the key factors influencing their efficacy in lithium batteries. High energy densities and low chemical potential of Li/Li⁺ batteries (− 3.04 v vs. SHE) enable the best energy storage technology. Exxon later successfully commercialised coin cells in 1977–1979 that used a TiS₂ cathode, a Li–alloy anode, and an organic electrolyte (LiClO₄–dioxolane). Numerous Li-free cathode materials were then the subject of in-depth investigation after then, such as TaS₂, MoS₂, TiS₂, VS₂, NbS₂ and CrS₂ [1]. Lithium dendrites can grow and have the ability to puncture the separator during cycling, which unfortunately increases the risk of explosion in secondary batteries with a Li-metal anode. A carbon anode was used in place of the Li-metal anode to produce robust and secure Li-ion batteries (LIBs). Next-generation rechargeable Li and LIB batteries must currently charge more quickly, have higher energy densities, and be less expensive and safer to compete in the rapidly expanding industry. As a result, advanced LIBs are heavily researching and developing battery materials with high capacity. Unfortunately, many of the cathode materials utilised today for conventional LIBs, such as lithium cobalt oxide, lithium nickel oxide and LiMn₂O₄, have limitations in electrochemistry, including low energy density, safety risks, high cost and environmental concerns. The potential advantages of Li-free cathode materials, including as their affordability, high capacity and environmental friendliness, have sparked a boom in preparation work in recent years. Conversion-type metal fluoride cathodes, which have a low cost and a large theoretical capacity, are one sort of Li-free cathode material that has gained a lot of attention lately. Although S cathodes still have a lot of issues, such as low volumetric energy density and intermediate lithium polysulfide dissolution in the electrolyte, Li–S battery systems have mostly been explored. When compared to its counterparts in the oxide and sulphide family, the M-F bond’s high ionicity favours a greater reaction voltage [2]. NiF₂, FeF₂, FeF₃ and other metal fluoride-based cathodes, therefore, have great promise for energy storage applications. Theoretical operational voltages and specific capacities for Li-free cathodes are depicted in Fig. 1A [2]. Due to F⁻ s high electronegativity, it is discovered that metal fluorides exhibit the strongest electromotive force. When compared to LIB cathodes like LiNi_0.5Mn_1.5O₄ and sulphur, metal fluoride cathodes have high volumetric (> 6700 Wh L⁻¹) and gravimetric (> 1600 Wh kg⁻¹) energy densities (Fig. 1B, C). The cell energy densities of the transition metal fluorides are shown in Fig. 1D, with FeF₃ measuring 1125 Wh kg⁻¹ (1782 Wh L⁻¹) and CuF₂ measuring 1172 Wh kg⁻¹ (2178 Wh L⁻¹). It is important to note that the real energy densities will fall since the cathodes and anodes require a higher proportion of carbon and solid-state electrolytes to improve ionic and electron conductivity and generate superior battery performance. However, it is still significantly greater than the LIBs in use today (150–300 Wh kg⁻¹) [3] (Fig. 1).

Fig. 1

A Energy densities of Li-free cathodes (LiNi_0.5Mn_1.5O₄, Li(NiMnCo)1/3O₂) presented gravimetrically/volumetrically (B)/(C). Also, depicts solid-state Li battery energy density via FeF₃, D CuF₂, MnO₂ and S cathodes (inset) [3]

The main difficulties that metal fluoride cathode materials face are outlined in this review, along with generic solutions that can be used in cells. Numerous techniques can be used to produce these materials, and their electrochemical uses as cathode materials for LIBs and Li batteries are being researched. The benefits and drawbacks of using metal fluorides as electrode materials are addressed last but not least. We offer a thorough analysis of current developments in the electrochemistry of Li battery cathode materials made of metal fluoride.

The reduction of the active ingredient into metallic nanoparticles is one of the chemical processes used in batteries and the production of a Li compound is conversion reactions, as opposed to intercalation and alloying reactions. The generalised reaction formula can be written as

$${\text{M}}a{\text{X}}b\, + \,\left( {b \, \cdot \, n} \right){\text{Li}}\, = \,a{\text{M}}\, + \,b{\text{LinX }}\left( {M\, = \,{\text{metal}};{\text{ X}}\, = \,{\text{O, S, F or P}}} \right).$$

(1)

It should be noted that when M is electrochemically active with Li (such as Sn, Sb or Zn), an alloying reaction may occur; however, this is outside the purview of this review. Due to the fact that utilising metal compounds with high oxidation states can boost the specific capacity, conversion-type electrode materials can typically attain large reversible capacities [4]. In addition, these materials’ functioning potential may be easily adjusted by tuning the ionicity of the M–X bond.

FeF₃ cathodes with the multi-electron transfer mechanism have drawn the most attention amongst all metal fluoride phases because of their high energy density and inexpensive price. During the conversion event, this substance transforms into nanocomposites of metal nanoparticles spread in a LiF matrix. On the other hand, during lithiation, freshly created LiF occupies the space around the metallic iron nanoparticles, whereas metallic iron may form nanoparticles of Fe at the same beginning atomic sites in the old phase. The ionic and electronic conductivities of the new phase, the interfacial energies and the anions’ and cations’ diffusion coefficients can all affect the morphology of these cathode materials. Numerous investigations have linked reaction kinetics and the phase behaviour in the insertion regime, although using various techniques [5]. The bulk of investigations show that the trirutile Li_xFeF₃ phase with an edge-sharing structure develops from the ReO₃ structure made up of corner-sharing FeF₆ groups. Despite having differing x values, there is a transition that significantly alters the Fe ordering and anion packing. FeF₃ lithiation is primarily a diffusion-controlled substitution mechanism, according to research by Hua et al. In addition, there is a distinct topological interaction between the metal fluoride and F sublattices and that of LiF [6]. During the initial lithiation of FeF₃, FeF₂ is generated on the particle surface. Then coupled with FeF₂, the A-LixFeyF₃ phase (cation ordered and stacking disordered) is generated. This structure is related with α-/β-LiMn²⁺Fe³⁺F₆ and can topotactically transform to B- and then C-Li_xFe_yF₃ before forming LiF and Fe. The reaction pathways of FeF₃-FeF₂ cathode are represented in Fig. 2.

Fig. 2

Reaction pathways of FeF₃–FeF₂ cathode. The reference phases in the phase diagram are indicated by light blue circles to show the positions of A- and B-Li_xFeyF₃, whose Fe concentration is off-stoichiometric [6]

Through colloidal synthesis, Xiao et al. produced single-crystalline, monodisperse FeF₂ nanorods [7]. Using an ionic liquid electrolyte (1 M LiFSI/Pyr1,3FSI), the nanorods showed good cycle stability and nearly the predicted capacity of 570 mA h g⁻¹. The conversion procedure explained in this study reveals that several processes control the discharge and charge responses. According to Fig. 3, the charge mechanism is an interface-controlled reaction in which Fe²⁺ slowly diffuses through a lattice of rock salts. This is a major factor in the voltage’s sluggish natural rise. Fe⁰ quickly diffuses across open channels during the discharge process, which is a diffusion-controlled reaction. Geographically and chemically symmetric charge and discharge pathways leads the study to hypothesise that voltage hysteresis is predominantly caused by the reaction overpotential [7]. Based on the aforementioned findings, materials design may be able to reduce response hysteresis. For the reversible conversion reaction, considerable contact between nanosized products can be achieved using doping or structural control to provide a conductive bridge for electron transport. Metal fluoride materials have a number of theoretically significant features, but in practise, these materials need to get over a few hurdles to behave as intended. These issues, which also include low ionic and electron conductivities, voltage hysteresis and unfavourable side reactions between active materials and electrolytes, may lead to low Coulombic efficiency and energy efficiency.

Fig. 3

(1) Surface forms disordered Fe/LiF. (2) Interior forms rocksalt/trirutile phases, with Fe/LiF particles having double-layered shells. (3) Shell confines [001] spread, demarcating converted and unconverted regions. (4) Fluoride matrix generates Fe nanoparticles. (5) Charging consumes Fe nanoparticles. (6) Pseudo-single-crystalline rutile nanorod forms, followed by double-layered shell conversion [7]

Limitations of Conversion Materials and Strategies

Metal fluoride conversion cathode are good cathode materials for lithium battery. But they are still unable to be used industrially due to a number of technological limitations. These drawbacks include substantial voltage hysteresis, low electrical conductivity, dissolution of active materials [8], volume change during cycling, irreversible structural changes [9] and unfavourable interactions with electrolytes [10]. Metal fluoride electrodes are inappropriate for use in practical energy storage devices due to a few drawbacks. First off, the highly insulated LiF created during conversion processes in LIBs makes cycling a challenge due to issues including high voltage hysteresis and poor electronic conductivity. Second, the SEI film may clearly degrade as a result of the volume changes that occur during lithiation and delithiation. The problems with metal fluoride electrode materials have been addressed in a number of ways, making them interesting options for energy storage devices. When metal fluorides are converted, complex chemical pathways, highly insulating products (like LiF) and connections with transition metals are frequently broken. Cycling issues and voltage hysteresis are caused by these properties. Even though few papers have shed light on these issues, additional in-depth illumination is anticipated from sophisticated characterisation and modelling methods. Further researchers using different methodology to overcome above mentioned limitation to enhance the efficiency of the metal fluorides, some of their efforts are highlighted below.

Low Conductivity

Metal fluoride (FeF₃ and FeF₂)-based cathode materials are particularly promising due to their substantial volumetric and gravimetric capabilities. Metal fluoride compounds are not good electrical conductors because of the large band gap caused by the strong ionic connection between M and F. Massive metal and LiF clusters are unable to develop during the cycling phase due to the high mass transport resistance brought on by the high interfacial energy at the Fe/LiF contact. In addition, the metal fluoride cathode’s abrasive interactions with the electrolyte increase cell resistance and help with the breakdown and dissolution of Fe, which results in low-rate capability and quick capacity fading during cycling as well as irreversible structural changes. The creation of various metal fluoride-based nanocomposites has received considerable attention in an effort to address some of these issues [4]. Many conductive carbons, including graphene, carbon black, carbon fibres, carbon nanotubes (CNTs), and microporous and mesoporous carbons, have been extensively researched and have shown appreciable gains in battery efficiency in such hybrid composite cathode syntheses [11].

This limitation can be overcome and the reversible capacity and durability of metal fluoride electrodes can be advantageously increased using highly conductive materials like carbon and metal/metal oxides. This method can be used to address the electrode’s poor electronic conductivity. By including conductive species with an electron transport chain to the active core material, the kinetic behaviour can be effectively improved whilst maintaining good electrical contact. In addition, the side reaction is effectively blocked by the coating or attachment of external moieties on the active substance’s surface, outweighing dissolution.

Carbon-Based Metal Fluoride Nanocomposites

Carbon-based nanocomposites have been employed successfully because of their special properties, which include great structural stability, high electronic conductivity, big pore volume and high surface area. Consequently, carbon composites with nanocomposites as their primary constituents have improved electrochemical performance. Equation (2) states that the duration of Li-ion diffusion, t, is inversely correlated to the square of the diffusion length,

$$L{:}t \, = \, L2{/}D,$$

(2)

where D is the Li-ion diffusion coefficient. To improve the electrochemical effectiveness of metal fluorides, use reversible cathodes, carbon and metal fluoride nanocomposites (CMFNCs) were initially described by the Badway group [10]. By ball milling, they created the CMFNCs, which displayed a 600 mAh g⁻¹ of reversible specific capacity, with a 4.5–1.5 V discharge voltage range. The presence of numerous surface defects on the nanoscale crystals’ surfaces, which significantly increase the ionic and electronic activity, leads to this conclusion. The remaining specific capacity was provided by a two-phase conversion reaction at 2 V, whilst the cathode reduction of Fe³⁺ to Fe²⁺ between 3.5 and 2.8 V produced almost a third of the discharge capacity. In the discussion of the electrochemical performance of the metal fluoride-based conversion process, the early findings of CoF₂, NiF₂ and FeF₂ CMFNCs were also used for comparison. Despite significant efforts, the partial reaction irreversibility and limited cycling stability of metal fluoride cathodes have mostly proved a barrier to their application [11]. Consequently, it would be advantageous to find a different way to synthesise cathode materials. Because of this, intercalating metal fluorides into a carbon matrix may be a useful strategy for enhancing cycling performance. In example, carbon electrode materials with metal oxide encapsulation have shown improved cycling capabilities in LIBs [12]. These theories propose that the addition of metal fluoride may improve the electrochemical properties of carbon materials such as graphene, carbon fibres, CNTs and mesoporous carbon materials. By pyrolyzing a combination of LiF and ferrocene at 700 °C in an Ar atmosphere, Prakash et al. developed a ferrocene-based carbon-iron and LiF nanocomposite [13]. The composite was made up of multi-walled CNTs and graphite structures resembling onions, with LiF scattered throughout the carbon matrix and Fe₃C and Fe nanoparticles inserted therein (Fig. 4A, B). The nanocomposite has 82 m² g⁻¹ of specific surface area and 0.14 cm³ g⁻¹ of mesopores as well as 0.025 cm³ g⁻¹ of micropores. The built cathode has a potential range of 0.5–4.3 V and a reversible specific capacity of about 300 mAh g⁻¹ at a current density of 20.83 mA g⁻¹ (Fig. 4C and D). It displayed great capacity retention (83.5% after 200 cycles) and a high-rate capability. Further research is needed to maximise its reversible capability because to its limited capacity. For example, uniform particle size/distribution, removal or reduction of Fe₃C, a decrease in carbon content, the acquisition of more Fe-encapsulated CNTs, etc. Another group produced FeF₃. By functionalizing CNT surfaces, nanoflowers are produced on cathode materials with FNCB-based nanoarchitecture. Utilised as cathode materials, the FNCBs demonstrated better electron transport and Li-ion storage.

Fig. 4

A After a charging test, TEM pictures of a nanocomposite electrode were taken. B The 300 K Mossbauer spectrum of a nanocomposite. C The specific capacity and cycle performance of the cathode. D Assess the prepared sample performance [13]

Amongst carbon-based materials, graphene and reduced graphene oxide (RGO) stand out for their exceptional mechanical and electrical conductivity. To provide simple electron paths, graphene sheets may be preferred conductive networks and construction units. For the first time, graphene-wrapped FeF₃ nanocrystals (FeF₃/G) were created as cathode materials using a vapour–solid technique (Fig. 5A) [14]. Due to the reduced electrical resistance and buffering effect of the graphene layers, the as-prepared FeF₃/G nanocrystals give an improved capacity of 155 mAh g⁻¹ at a current density of 104 mA g⁻¹ and exhibit greater rate capability and cyclic stability over 100 cycles (Fig. 6B, C).

Fig. 5

A FeF₃/G nanocomposite production process illustrated schematically. B FeF₃/G and bare FeF₃ voltage profiles and rate capabilities. FeF₃/G nanocomposite cycling performance and Coulombic effectiveness [14]

Fig. 6

A FeF₃∙0.33H₂O/rGO preparation procedures are shown schematically. B FeF₃∙0.33H₂O/rGO images from the SEM and TEM. At a current density of 500 mA g⁻¹, the cycling performance of FeF₃∙0.33H₂O/rGO and FeF₃∙0.33H₂O without rGO is shown in C. D Evaluate the performance of the FeF₃∙0.33H₂O/rGO and FeF₃∙0.33H₂O NPs [14]

Similar results were obtained when FeF₃∙0.33H₂O nanoparticles were seeded on RGO using an in situ method, according to Qiu et al. [15]. The chemical adjustment of the interfacial interaction between rGO and FeF₃∙0.33H₂O was thought to be responsible for these outcomes, which included increased cycle stability and high particle loading (Fig. 6A, B). Particularly, the FeF₃∙0.33H₂O/rGO nanocomposites exhibit exceptional cycle stability (133.1 mAh g⁻¹ after 100 cycles at 100 mA g⁻¹ with 97% capacity retention) as well as a higher discharge capacity of 208.3 mAh g⁻¹ at a current density of 0.5 C. Later, using a vacuum impregnation process, extremely homogeneous carbon-based nanocomposites containing nanoconfined FeF₂ were successfully created by intercalating a fluoride precursor with activated carbon powder. When a cell is operating, the carbon pore walls feed to the electrochemical reaction sites as holes or electrons, accommodate fluoride volume variations during lithiation and delithiation, and prevent the physical separation of fluoride particles. When carbon nanopores are used to enclose metal fluoride nanoparticles, the resulting nanocomposite exhibits noticeably better cycle stability and electrochemical efficiency in LIBs.

The focus of prior studies was on putting FeF₃ particles into the nanopores of RGO, CNTs and mesoporous carbon or onto the surface of carbon matrices. However, the creation of carbon matrices is costly and calls for advanced synthetic methods. In addition, it is difficult to achieve close contact between carbon matrices and FeF₃ nanoparticles, making it difficult to prevent a larger fraction of FeF₃ beyond the inner pores [16]. However, a FeF₃-carbon composite with tight contact between the conductive carbon matrix and FeF₃ nanoparticles has been made by altering the carbonization and polymerization processes. This method efficiently enhances the intimate contact between carbon and FeF₃ by fully mixing organic precursors (which can be converted into conductive carbon) with FeCl₃. Thus, composites of FeF₃ nanoparticles enclosed in graphitized carbon were produced using citric acid (C₆H₈O₇), FeCl₃ and ethylene glycol as carbon sources, iron precursors and crosslinking agents, respectively. The initial discharge capacity of the produced graphitic carbon-coated FeF₃ composite is 188 mA h g⁻¹. The capacity in the 1.5–4.5 V voltage range eventually reached 421 mAh g⁻¹ after protracted activation operations, which is larger than bare FeF₃ [14]. The increased cycling and rate performance of this nanocomposite was principally brought about by the controlled size of the FeF₃ particles and the conductive graphitic carbon layer on the surface of the FeF₃ across lengthy cycles.

Different organic compounds have been thought of as electrode materials in the past, but they have not gotten as much attention as inorganic materials have because conductive polymers (CPs) were developed and intercalation compounds, like LiCoO₂, were made possible as cathode materials for LIBs in the 1980s. Organic electrode innovation is currently required due to the inherent limitations of intercalation compounds as LIB cathodes. As a result, carbonyl compounds are typically a superior electrode material than CPs and provide excellent potential for developing cost-effective, eco-friendly and practical energy storage systems. As a result, by intercalating RGO with spherical Li₂C₆O₆ under morphological control and then altering with a FeF₃ covering, a hybrid FeF₃@Li₂C₆O₆/rGO nanocomposite was produced.

The FeF₃@Li₂C₆O₆/rGO hybrid cathode, when used as constructed, demonstrated higher cycling performance (320 mAh g⁻¹ after 100 cycles) [17], which was attributed to the Li₂C₆O₆’s surface being coated with electrochemically active FeF₃, which was able to successfully prevent Li₂C₆O₆ dissolution in the electrolyte. This interface engineering methodology and morphological control make up a practical and scalable method for greatly enhancing the rate performance and cycle characteristics of organic-based cathode materials. Reddy et al. recently created iron pentacarbonyl-[Fe(CO)₅] and fluorinated carbon to easily create carbon-FeF₂ nanocomposites in one step at 250 °C. As a result, the electrochemical characteristics of the resulting C-FeF₂ nanocomposites, which serve as the cathode, are enhanced. By treating Fe(CO)₅ with four different CFx precursors, they were able to produce four separate C-FeF₂ nanocomposites made of carbon black, petcoke, carbon fibres and graphite. The good electrochemical performance for high energy density LIBs was also evaluated at 25 and 40 °C.

To create flexible, free-standing cathodes out of metal fluoride-carbon nanofiber nanocomposites, Fu et al. reported a successful and adaptable method. The as-synthesised constructed FeF₃-C/Li cells have exceptional stability (> 400 cycles without structural degradation) and a greater discharge capacity of 550 mAh g⁻¹ at 100 mA g⁻¹. These promising properties can be directly linked to the durability of the electrically conductive carbon network, the nanoconfinement of FeF₃ nanoparticles, the decrease in irreversible separation and aggregation, the development of ultrafast pathways for electron transport and ion diffusion, and the inhibition of unfavourable reactions between the liquid electrolyte and active materials.

The composite design of active materials is another successful and popular technique to improve the electrochemical characteristics. Carbon-based metal fluoride composite electrodes are always the best option. For instance, carbon nanofibers containing FeF₂ nanoparticles can stop the side reactions between FeF₂ and the electrolyte, improving the electrode’s electronic conductivity and enabling cycle stability of more than 400 cycles. According to previously published studies on battery electrode materials, the composite architecture design is a very effective strategy for preventing interactions between the active materials and electrolytes and achieving rapid transport in the electrode. For electrode self-protection and achieving fast electron transport, it has been challenging to combine nanosized FeF₂, FeF₃ and CoF₂ into a three-dimensional (3D) carbon matrix.

In addition, cathodes can profit from the 3D architecture design’s improved cycling stability, high active mass loading and high-rate performance. As a result, Wu et al. developed a FeF₃@C composite with a 3D honeycomb architecture utilising a simple and basic method. The isolated FeF₃ particles, which were evenly distributed throughout the 3D carbon honeycomb carbon framework and ranged in size from 10 to 50 nm, provided sufficient conduction pathways for achieving fast ion diffusion and electron transport in FeF₃ cathodes. The as-prepared 3D honeycomb architecture FeF₃@C composite cathodes demonstrated exceptional cycle stability within 1000 cycles, with notable capacity retentions and an unmatched rate capability of up to 100 C, (95–100%) within 200 cycles at 2 C. They also had high areal FeF₃ loadings (2.2–5.3 mg cm⁻²) [18]. It was demonstrated that a 3D architecture with a honeycomb topology is an effective composite design technique for metal fluorides that may be used to produce Li batteries with exceptional electrochemical performance.

Overall, the results are very encouraging in terms of the potential to enhance the structure and composition of carbon-based metal fluoride cathodes to enhance their rate performance and cycling stability. The electrochemical performance of carbon-based metal fluoride cathodes must still be improved to reach the standard required for commercial applications. This requires novel fabrication techniques that can precisely control the overall morphology and the nanoscale features of the carbon matrix and active materials [19]. To overcome these obstacles and allow for commercialization, more work is, therefore, required.

Metal/Metal Oxide-Based Fluoride Nanocomposites and Effect of Doping

In the recent years, doping has drawn a lot of interest from researchers studying intercalation-type electrode materials. Doping may also have definite benefits for conversion-type electrode materials by improving cathode performance through the intercalation of vacancies, ions, or heterogeneous/homogeneous metals. Ionic and electronic conductivities of the electrode materials will be enhanced as a result. In addition, a number of scientists have stressed the potential for reducing the cluster size and developing “true” conversion electrode materials during the lithiation process due to the decrease in interfacial energy and changing of the ion mobility of the clusters. By reducing the routes for mass transfer, the affinity of the gradual and steady separation of the cluster size may be decreased, and the voltage hysteresis for the charge/discharge may be reduced. This might improve the electrodes’ cycle stability and energy efficiency. In numerous recent articles, the electrode materials’ electronic conductivities were significantly enhanced by doping with silicon, producing outstanding discharge capacity utilisation.

To enhance its electrochemical characteristics, molybdenum bisulfide was ball milled with ferrous trifluoride. As a cathode material in LIBs, the orthorhombic-structured FeF₃/MoS₂ with a consistent shape demonstrated outstanding electrochemical performance. The material had a 170 mAh g⁻¹ initial discharge capacity at a 0.1 C rate with a voltage range of 2.0–4.5 V, and after 30 cycles, it still had 83.1% of that capacity. Wu et al. [4] used HF, FeCl₃, and NaOH as precursors in a liquid-phase process to create orthorhombic-structured FeF₃. To create a FeF₃/V₂O₅ nanocomposite, the resultant product was further ball milled with conductive V₂O₅ powder. After the V₂O₅ was added, the produced material’s electrochemical characteristics significantly improved. The FeF₃/V₂O₅ nanocomposite performed well in terms of rate and cycle. After 30 cycles, a discharge capacity of 209 mAh g⁻¹ was reached and maintained at a retention rate of 0.1C and a voltage range of 2.0–4.5 V.

Zhang et al. produced a spherical TiO₂-solvothermally coated FeF₃._0.33H₂O cathode material. The nanosized FeF₃._0.33H₂O was coated with a homogeneous layer of TiO₂ with an average particle size of 1.0 m and good dispersion ability. The initial reversible capacity, with a voltage range of 1.5–4.5 V, was 654 mAh g⁻¹ for discharging and 522 mAh g⁻¹ for charging at a retention rate of 0.1 C. After 200 cycles, the cathode made of nanomaterials maintained an excellent stability of 264 mAh g⁻¹, suggesting that the TiO₂ layer on iron fluoride might work as a LIB cathode material in the future. Using finely structured FeF₃ as a precursor, a new core–shell FeF₃@Fe₂O₃ nanocomposite (100–150 nm) and tunable iron oxide (Fe₂O₃) were produced with a straightforward heat treatment. The electrochemical performance of the as-prepared FeF₃ and core–shell FeF₃@Fe₂O₃ nanocomposites was investigated. The electrochemical performance of virgin FeF₃ and FeF₃@Fe₂O₃ nanocomposite was compared, and the in situ coating of Fe₂O₃ resulted in a substantial improvement even when the coating amount was 0.6–5.2 wt% [20].

Metal doping increases the ability of metal fluoride cathode materials to charge and discharge. The Li-ion diffusion coefficient can be raised by the doped metal, which also enhances the electrode materials’ electrochemical performance. Recently, a liquid-phase technique was used to create copper-doped FeF₃ nanomaterials. The addition of acetylene black by a mechanical ball-milling method substantially improved the electrochemical reversibility of Fe_1 − xCo_xF₃ (x = 0, 0.03, 0.05 or 0.07) materials. This resulted in Fe_1 − xCo_xF/C nanocomposites. With the addition of cationic Co, it demonstrated a noticeably better electrochemical performance. At retention rates 1, 2 and 5 C, the discharge capacities are 150, 140 and 125 mA h g⁻¹, respectively, and they retained their capacities for up to 92.0%, 92.2% and 91.7% of 100 cycles. Metal fluoride cathode materials may experience particle breakage whilst lithiating or de-lithiating in LIBs. For the purpose of improving the stability of the cathode nanoparticles and resolving this problem, Zhang et al. did Co doping for FeF₃ in LIBs [17]. This study successfully carried out calculations for Co doping onto the hexagonal tungsten bronze structure to produce Co_xFe_1 − xF₃ systems (x = 0.08, 0.17, or 0.25). The study’s results, which showed a sharp decline in the band after Co doping, may be due to the presence of Co 3d impurity energy levels between the conduction and valence bands. As a cathode material for LIBs, thermally stable Co-doped iron fluoride (Fe_0.9Co_0.1F₃._0.5H₂O) was created using a non-aqueous precipitation technique. The thermogravimetric measurement reveals that the cathode nanomaterial remained stable up to 243 °C, after which the elimination of water caused the crystal structure to collapse. After 200 cycles, the Co-doped cathode material still had a reversible capacity of 150 mAh g⁻¹, demonstrating a high discharge capacity (227 mAh g⁻¹) during cell cycling [21].

Huang et al. discovered that adding NiF₂ nanoparticles to FeF₂ cathode materials significantly improved their electrochemical performance. The results showed that FeF₂-NiF₂ nanoparticle form and composition could be precisely controlled, and that the electrochemical cell’s capacity fading was unintentionally increased. In a different study, binary NiF₂ nanomaterials’ electrochemical performance was examined when Ni was replaced with Cu for LIBs.

Using in situ TEM, the structure of different ternary metal fluorides with Cu substitution was studied. The results showed that the areal expansion during the first lithiation was reduced when the Cu substitution was between 1 and 25 weight percent. The reversible reaction significantly reduced the fluorine loss during delithiation, proving that Cu is a more advantageous replacement to improve the electrochemical performance. Recently, Li et al. [22] used ternary metal fluorides as cathode materials in LIBs, including AgCuF₃ and Cu_xFe_1 − xF₂. Thin-film Cu–Fe–F (CFF) cathode materials were successfully manufactured on-site by means of pulsed laser deposition. The reversible structural rearrangement that took place after delithiation, which was aided by high-resolution TEM, in situ XPS and specific electron diffraction techniques, can be attributed to the high specific capacity of 420 mAh g⁻¹ and good cyclic stability of the CFF cathode material as-prepared.

From the detailed literature survey, it is evident that the combination of metal fluoride and metal oxide in a composite strategy is a promising approach to enhance the electrochemical performance of cathodes in advanced battery technology. This strategy can be particularly useful for lithium-ion batteries, sodium-ion batteries or other energy storage systems. Here is how this strategy can improve cathode performance: enhanced energy density: metal fluorides, have high theoretical energy densities. Combining them with metal oxides increases the overall energy density of the cathode materials, which can result in batteries with higher energy storage capacity [23].

Similarly, the incorporation of metal fluoride in composites enhances the redox reversibility. Generally, metal fluoride materials often suffer from poor cycling stability due to large volume changes during charge/discharge cycles [17]. Metal oxides can mitigate this issue by providing structural support and preventing the pulverisation of the metal fluoride particles. This helps maintain the redox reversibility of the cathode material, leading to longer cycle life [24].

Furthermore, incorporation of metal oxide along with fluorides enhances the conductivity of the cathode, which is crucial for the efficient charge and discharge of the battery. This results in better rate performance and lower internal resistance. Metal fluorides are known for their good thermal stability and safety characteristics. Incorporating them into composite cathodes can enhance the overall safety of the battery. Hence, composites of metal fluoride along with metal oxides are playing a vital role in designing advanced battery electrodes.

Voltage Hysteresis

Between the charge and discharge potentials in rechargeable batteries, hysteresis is a typical issue that is related to thermodynamic and kinetic parameters. It could result in lower capacity utilisation and poor energy efficiency. In addition, a bigger voltage hysteresis in the charge and discharge window could result in additional side interactions between the active ingredient and the electrolyte and potentially unstable SEI. Sadly, MF cathodes exhibit unfavourably high voltage hysteresis, which is frequently brought on by the materials’ poor electronic conductivity, the battery’s quick deterioration and variations in surface/interfacial energy throughout the conversion reactions. Another major cause of voltage hysteresis is energy losses for correcting the activation energies of breaking chemical bonds. Fe–F bonds are broken during the discharge (lithiation) process of MF cathodes, resulting in the formation of lithium-containing lower free-energy molecules. Due to the significant difference in electronegativity between Li and F, the discharge process produces thermodynamically stable compounds like LiF, whose bonding energy is 577 kJ mol⁻¹. To break the stronger matching chemical bonds and disintegrate LiF, the charge process (reverse reaction) is necessary. Due to the high activation energy barrier introduced, there is a significant overpotential or voltage hysteresis. The misunderstanding that cells based on conversion MF cathodes, such as Li–CuF₂ battery systems, are just primary batteries is also a result of high voltage hysteresis. To associate the voltage profile with intermediate phases of FeF₃, Li et al. recently studied the evolution and geographical distribution of intermediate phases throughout the discharge–charge process. The results show the symmetric phase evolution of the electrode during cycling. The spatial growth of the electrochemically active phase, which is controlled by reaction kinetics, is different. They found that the kinetics of the FeF₃ electrode in nature is what causes voltage hysteresis. It originates from the Ohmic voltage drop, overpotential, and varied spatial distributions of active phases, as shown in Fig. 7. Therefore, the substantial hysteresis can be reduced through sensible material and electrode microstructure optimisation.

Fig. 7

Phase progression in the electrode during discharge and charge is shown schematically [25]

For conversion-type materials, using elemental doping is a viable solution to the voltage hysteresis issue. The addition of homogeneous/heterogeneous metallic (non-metallic) ions or vacancies improves the electrode material’s ionic and electronic conductivities. FeF₂ and FeF₃ doped with Co, for instance, displayed decreased voltage hysteresis. Over 100 cycles (for this chemistry), FeF₃ exhibits remarkable cycle performance, and the reaction kinetics are significantly improved [25]. To properly utilise the Cu²⁺/Cu⁰ redox spectrum in the application of copper-based fluorine rechargeable Li batteries, Wang et al. have proposed a new way for the first time.

The prepared FeF₂ (tetragonal rutile) and CuF₂ (monoclinal distorted rutile) solid solutions gave Cu_1 − xFe_xF₂ a tetragonal rutile structure with high symmetry. It involves a two-stage lithiation process during the discharge process of the Cu_1 − xFe_xF₂ cathode. The first stage is at an upper plateau of ~ 2.9 V, which is related with a Cu-based conversion reaction, in a similar potential range as CuF₂. The second stage is at a higher potential of ~ 2.2 V, which is related with a Fe-based conversion reaction [23]. Cu_0.5Fe_0.5F₂ has an initial discharge–charge process reversible capacity of 543 mAh g⁻¹. The reduced voltage hysteresis and lack of the voltage drop observed in the Cu_0.5Fe_0.5F₂ electrode are the primary causes of the better dynamic performance of as-prepared cathode materials. After first-half conversion, FeF₂ shrinks and the lattice disorder intensifies, which is what eliminates the voltage loss and results in a higher discharge voltage during the first second-half conversion. Cu²⁺/Cu⁰ voltage hysteresis is limited because of the low nucleation barrier for the formation/decomposition of the Cu–F bond.

The reduced voltage hysteresis in the context of metal fluoride cathodes in lithium-ion batteries is indeed related to lower nucleation energy, but it is primarily a result of the unique electrochemical properties of iron fluoride and its structural transformations during cycling. Metal fluoride is a promising cathode material for lithium-ion batteries due to its high theoretical capacity, which can potentially result in higher energy density. The reduced voltage hysteresis typically occurs when the structural changes of iron fluoride during lithiation (charging) and delithiation (discharging) are minimal [26]. In other words, the metal fluoride cathode maintains a more stable and reversible crystal structure during the lithium insertion and extraction processes. This reduced structural change corresponds to lower nucleation energy for lithium nucleation and growth. When the structural changes are minimal, it means that the phase transitions between different metal fluoride are minimised. Such transitions can consume energy and result in higher voltage hysteresis. Lower nucleation energy, along with a stable crystal structure, allows the battery to operate more efficiently with reduced energy losses [27]. Researchers have been working on various strategies to reduce the voltage hysteresis in iron fluoride cathodes, such as nanostructuring the material, introducing conductive additives, and optimising electrolyte formulations. These approaches aim to maintain the structural stability of iron fluoride, minimise phase transitions, and improve the overall performance of iron fluoride-based lithium-ion batteries.

Side Reactions with Electrolytes

Because they increase voltage hysteresis, diminish the Coulombic efficiency and stability of the cathode SEI, and compromise safety, unfavourable side reactions between electrodes and electrolytes are detrimental. The SEI serves a variety of functions, one of which is to restrict electrolyte-cathode (or anode) side reactions. Great cation conductance and electrical resistance, a few nano-metre in thickness, great mechanical toughness, and stability across a broad voltage range are all characteristics of an excellent SEI layer. Regardless of whether an ion insertion anode is used or not, a stable SEI is a recognised requirement for safe battery operation. Due to their low electrochemical potentials (1.5–4.0 V vs. Li/Li⁺), conversion-type cathodes should contribute to the thermodynamic stability of electrolytes. However, some cathode chemicals and certain salt anions, especially in concentrated electrolytes, might alter the oxidative stability of electrolytes. In addition, the electrolyte may be reduced at the cathode surface since the conversion cathode is regularly discharged at a low voltage compared to Li/Li⁺ (rarely down to 1.2 V) [28]. The different electrolyte reduction products can be oxidised more quickly than the original electrolyte at moderate cathode potentials.

The reduction reaction might also be catalysed by a few conversion cathode species. For instance, it has been discovered that the creation of a lithium carbonate species, which is sensitive to reduction catalysed by nanometals, influences the cyclic stability of MFs following battery discharge to 1.2–2.0 V. Due to the constant high-volume variations experienced during cycling, conversion-type cathodes with weak cathode SEI may not be able to stabilise. Contrarily, the development of a stable cathode SEI might also aid in stabilising the conversion cathode and averting unfavourable interactions between the cathode and electrolyte.

To overcome the limits of cathodes based on the conversion reaction, modifications have been made to the composition of lithium salts, organic solvents, salt concentrations and additions of organic and inorganic electrolytes in the case of liquid electrolytes. Classic organic carbonate esters including ethyl carbonate, propylene carbonate and ethyl carbonate are frequently employed as solvents for intercalated cathodes and graphite anodes. However, the carbonate solvents could have some negative effects if the cathode is only partially converted. Recently, a LiTFSI salt-rich electrolyte has been developed to address this problem. By creating a protective layer in situ on the electrode surface, it protects cathode materials from dissolving. On the other hand, it was asserted that the structure of the fluorine-containing layer lessened the side reaction between fluorine and the electrolyte. In contrast, atomic layer deposition was used to create a fake SEI layer that was extremely thin.

For instance, Kim et al. synthesised 3D Al₂O₃-coated FeF₂ nanoparticles on Ni support. The outcomes showed that the ion transport channel considerably enhanced the electrochemical applications built into the electrode. The 3D FeF₂ electrode’s initial discharge capacity was 380 mAh g⁻¹ at 200 mA g⁻¹ [6]. The 3D support shortened the Li-ion diffusion channel, which improved reaction kinetics and specific capacity, and created a reliable platform for electron transfer. The SEI should, therefore, receive more focus to greatly boost the cyclic stability of metal fluorides.

Synthesis of Metal Fluorides

The disclosed results indicate excellent possibilities for enhancing the composition and design of metal fluoride cathodes to increase battery performance. But to optimise the cathode chemistry performance to the point of practical applications, unique synthesis procedures must be further developed to control the size characteristics and morphology of the conversion-type cathode materials. Different chemical and physical techniques have been used to prepare metal fluoride electrodes. Based on the unique synthesis technique used to create nanoscale architectures, the metal fluoride cathode materials’ electrochemical performance has improved dramatically. By uniformly mixing with a highly conductive material or by lowering the particle size, the low ionic and electronic conductivities of metal fluorides can be enhanced because nanoparticles can hasten Li-ion diffusion and have a large surface area for interaction with electrolytes. In addition, the tension and strain brought on by volume fluctuations can be resisted or stopped by nanostructured electrode materials. It has been demonstrated that the specific capacity and cyclic stability of the electrode materials are significantly influenced by the precise nanoscale form. This section introduces the processes used to create electrode materials, such as hydrothermal, solvothermal, microwave, vapour–solid, ion, and sol–gel processes.

Hydrothermal Methods

Due to the advantages of the crystalline powder at high temperatures, hydrothermal techniques are frequently used in synthetic chemistry. The hydrothermal process’s regulated reaction conditions produce the required shape. Using this method, a product with good dispersion, high purity, uniform morphology, and regulated particle size is produced. Monocrystalline solids, as well as 2D and 3D materials, with particle sizes ranging from microns to nanometers, can be synthesised using it [21]. The low electronic conductivity of LIBs was improved by the inclusion of ordered mesoporous carbon (CMK-3) in the FeF₂ cathode nanomaterials. The hydrothermal treatment of CMK-3 and iron oxide resulted in the production of the FeF₃ precursor after a topochemical reaction. Figure 8A shows the hierarchical conductive FeF₂/CMK-3 nanoparticle network following further annealing of the FeF₃ precursor. This style of hierarchical structure offers the network constant electronic conduction as well as porosity for prepared material volume growth. The created cathode material displayed a consistent cycle life of 1000 cycles with a 0.3% capacity reduction per cycle mA g⁻¹, which after 100 cycles were 500, 400 and 320 mAhg⁻¹, respectively. Figure 8B displays the substantial discharge capabilities at current densities of 500, 2000 and 4000.

Fig. 8

A Porous FeF₂-CMK-3 composite synthesis is shown schematically. B FeF₂/CMK-3 cathode electrochemical performance: a 200 mAg⁻¹ charge–discharge profiles; b and c cycling performance [21]

Solvothermal Methods

Solvothermal techniques were essentially built on the foundation of hydrothermal techniques, with the distinction that water is not employed as the reaction medium. One or more of the precursors of the reaction dissolve under intense pressure in a solvothermal reaction. Short reaction durations, quick response kinetics, homogeneous particle dispersion and high crystallinity are only a few benefits of this technique. Solvothermal processes have improved the electrochemical performance of electrode materials, including high-rate performance and long cycle life. The nanostructure and composition of FeF_3·0.33H₂O hollow nanospheres in solvothermal reactions are influenced by Fe and F sources, reactant concentration, solvent composition and reaction time. These hollow nanospheres exhibit excellent performance as lithium-ion battery cathode materials due to their high reversible capacities and cycling stability, driven by the beneficial impact of the hollow structure and mesoporous features, enhancing electrolyte and lithium-ion diffusion and electrolyte/electrode contact area [29].

In another study, hollow prismatic iron fluoride with a 0.1–0.5 μm thick wall was synthesised via solvothermal method. It offered initial discharge capacities of 106.7 mAh g⁻¹ and 60% retention after 100 cycles at 0.5 C. Also, the composites of iron fluoride with 15 wt% acetylene black (AB) resulted in FeF₃·0.33H₂O/C nanocomposites. These nanocomposites deliver 160.2 mAh g⁻¹ discharge capacity at 0.5C and maintain high initial discharge capacity of 137.5 mAh g⁻¹ at 5 C, with capacity retentions of 85.0% and 75.7% after 100 cycles at 0.5 C and 5 C, respectively [30]. In another study, scientists built an architecture of FeF₃·0.33H₂O, binder-free nanosheets array on Ti foil by solvothermal method. This unique structural design showed a capacity of 161.5 mAh g⁻¹ at 1 C and a capability of 117 mAh g⁻¹ with a current density of 4 Ag⁻¹ (20  C). Further they observed that incorporation of graphene quantum dots also enhanced the cycle performance of the battery 100 times. With this, the cycle performance greatly improved—even cycled over 1000 times at 2 C with the capacity attenuation rate as 0.03% per cycle [31] (Fig. 9).

Fig. 9

Overview of synthesis of FeF₃ composite from solvothermal method [32]

In a different investigation, solvothermal synthesis was used to create Fe_(1 − x)Co_xF₃/multi-walled carbon nanotube (MWCNT) nanocomposites. MWCNTs were used to encase the iron fluoride nanoparticles that had been doped with Co. The developed materials’ results showed that following Co doping, the crystal structure had been altered, the band gap had shrunk, and the Li-ion diffusion capacity had increased. In addition, the Fe_0.96Co_0.04F₃/MWCNT nanocomposites performed better electrochemically as a result of the MWCNT wrapping’s higher conductivity. The nanocomposites’ high specific capacity was 217.0 mAh g⁻¹ at a rate of 0.2 C between 2.0 and 4.5 V [33]. Comparing the electrochemical performance to that of the FeF₃/MWCNT nanocomposite equivalents, the former was better. The specific capacities of the two produced nanocomposites for Li storage in Li batteries decreased to 187.9 mAh g⁻¹ and 160.7 mAh g⁻¹ after 50 cycles, respectively, demonstrating their cyclic stability.

Microwave-Assisted Methods

In comparison to conventional synthetic techniques, microwave-assisted procedures represent potential avenues for the synthesis of metal fluoride nanoparticles. Since its initial description in the areas of synthetic chemistry and materials synthesis, method has rapidly gained popularity in this research area. This approach differs from others, such as the hydrothermal and solvothermal procedures, in that the reaction is totally dependent on the quick heating, which yields chemicals and solvents. In addition, the quick heated reaction has a high rate of reaction, is highly efficient and uses less energy. In recent years, the tailorable morphologies have been produced by combining room-temperature ILs with a microwave heating process. Hierarchical mesoporous self-assembled iron fluorides (HMIFs) were effectively produced using a microwave-assisted method. Figure 10 demonstrates the process and likely steps in great detail. A large number of nanorods measuring more than a dozen nanometers in length were used to create the dual nature fluorides FeF₃∙H₂O and Fe_1.9F_4.75∙0.95H₂O, and their electrochemical performance in LIBs was evaluated. The HMIFs were homogeneous in shape, had a 94 m² g⁻¹ specific area, a hierarchical mesoporous structure and an internally somewhat hollow core. After 100 cycles, the nanomaterials displayed good cycle stability, with a specific capacity of 148 mA h g⁻¹ at 0.1 C [18] when utilising any conducting agent, as well as a high reversible capacity of 200 mA h g⁻¹. In addition, the specific charge capacity at 5C was greater than 100 mA h g⁻¹, demonstrating the development of HMIF materials for LIB applications requiring strong electrochemical performance. The formation of a special structure that offered significant Li storage sites, a sizable electrode/electrolyte contact area, and consequently a volume change during lithiation/delithiation as well as a simple mass transfer of electrons and Li ions during the electrochemical cell reaction were credited with this exceptional performance.

Fig. 10

Diagrammatic representation of the formation process for partially hollow HMIFs. From step 1 to step 5, the following processes are performed: mixing the reactants; combining the solvated ions; first nanoparticle emergence; Ostwald ripening process and product completion [18]

Using sodium ethoxide, ammonium fluoride, and previously synthesised FeF₂ rutile colloidal particles, nanosized NaFeF₃ perovskite particles were produced. Figure 11 shows a schematic diagram, phases from powder X-ray diffraction, 3D models of the structure and the projected crystal structures for rutile and crystals of the orthorhombic variety. For storing Na and Li in energy storage devices, the perovskite NaFeF₃ nanoparticles demonstrated outstanding electrochemical performance with low polarisation [12]. According to a theoretical investigation using polymorphism, the stable corner-sharing cubic Li and Na perovskite with stable cycle, low volume change and thermodynamic cost has increased performance and outstanding capacity retention.

Fig. 11

A NaFeF₃ electrode material synthesised. B FeF₂ and NaFeF₃ XRD patterns as-prepared. C XRD-derived 3D structural models [12]

Sol–Gel Methods

Sol–gel technologies, a variety of synthetic chemical processes, can be utilised to make complex nanostructures with adjustable particle size, placement of nanopores, and uniform particle distributions. Polymers or other organic substances with activity can functionalize the nanoporous behaviour of the particle during gel formation, even in a dry environment. Nanoparticles can be generated inside the nanopores and employed in a variety of applications with good performance after being carefully heated and using the reaction precursor solution under temperature control. The sol–gel approach is distinct from other traditional methods used for nanomaterial synthesis due to a number of particular benefits. The first is the creation of a viscous solution as a result of the process-induced dispersion of the basic components. Even after the gel has been produced, a molecularly homogeneous solution of the reactants can be obtained, and a homogeneous mixture can be produced quickly. The second is that molecular-level homogenous doping is feasible since it is simple to incorporate more components throughout the reaction. The third is that it is simple to carry out the reaction, even at low temperatures, unlike a reaction requiring solid components. This is because solid particle diffusion occurs at the micrometre level for solid particles, whereas the reactants in the sol–gel process diffuse in the nanometer range. Low temperature is, therefore, required for the production of the nanoscale. Sol–gel synthesis was used to create FeF₃ nanocrystals with a particle size of 30 nm for LIB electrodes. The as-synthesised particles were made using RGO, which has a high specific retention of 150 mA h g⁻¹ [34] and stable cycle life for Li ions, even after 50 cycles, to improve the electrochemical performance. Another study created nanocomposites for LIBs based on LiF and FeF₂ using the sol–gel technique in ethanol. SEM and TEM characterisation of the LiF/FeF₂ composite showed that 10 nm nanosized LiF and FeF₂ crystals were formed, as shown in Fig. 12A, with a large surface area of 119 m2 g⁻¹ and the outcomes of EDX and XRD analysis. Mesopores were discovered due to their huge surface area and adsorption–desorption hysteresis, which may have a good effect on the electrochemical performance. According to data obtained for the nanocomposites and displayed in Fig. 13B, the reversible conversion process had a consistent cycle life and an initial discharge specific capacity of 225 mAh g⁻¹ at a current rate of 10 mA g⁻¹.

Fig. 12

A LiF-FeF₂ composite SEM images in a, b. Fe (c) and F (d) EDX mappings. LiF-FeF₂ electrode TEM images are shown in e, f. B Charge–discharge performance includes the following: a charge–discharge profiles at current densities of 10 mA g⁻¹; b cycling performance for the first 20 cycles at 10 mA g⁻¹ and c rate capabilities (from 10 to 1000 mA g⁻¹) [34]

Fig. 13

A Composite made of FeF_3∙0.33H₂O/GNS is synthesised. B Discharge profiles of the FeF_3∙0.33H₂O/GNS composite and C cycling performance of FeF_3∙0.33H₂O without GNS [36]

Iono-thermal Methods

In place of traditional solvents, ionic liquids (ILs) have been discovered. ILs are made up of cations and anions, one of which must be organically derived and have a melting point lower than a given temperature. Based on their melting points, this class of solvents can be split into two main groups: ILs that melt at room temperature and ILs that melt close to room temperature both have melting points < 100 °C [35]. Low vapour pressure is one of their distinguishing characteristics that sets them apart from other solvents. Good electrical conductivity, fire resistance, high thermal stability, the capacity to dissolve a variety of inorganic and organic substances, non-volatility, a broad temperature range and recyclable nature are amongst the primary properties of ILs. ILs made a viable solution for energy storage systems because of their properties. An IL-assisted technique was created for the in situ synthesis of FeF₃/GNS nanosheet hybrid nanomaterials. In addition to acting as sources of green fluoride, the purpose of ILs was to enable uniform dispersion and tight surface modification of FeF_3•0.33H₂O on graphene nanosheets. Because of the electron transfer path taken by the iron nanoparticles and GNS in LIBs, as illustrated in Fig. 13 along with the discharge profile at different currents, the FeF_3•0.33H₂O/GNS cathode nanomaterials as-synthesised showed a significant improvement in both specific capacity and rate performance. Due to the hybrid’s durable construction and solid graphene-iron fluoride nanoparticle connection, iron fluoride nanoparticles exhibit a noteworthy electrochemical performance of 115 mAh g⁻¹ after 250 cycles even at 10C. A different experiment used ILs as the reaction medium to create mesoporous FeF_3∙0.33H₂O cathode materials for LIBs at low temperatures [36]. Due to the design and optimal hydration of the water-induced microstructure at room temperature, the carbon-free FeF_3•0.33H₂O cathode materials in LIBs were expected to have improved electrochemical performance in terms of high reversible capacity and reactive voltage.

Vapour–Solid Methods

Fe₃O₄/graphene was thermally transformed into FeF₃/G cathode materials for LIBs using a vapour–solid method. This technique has been utilised to create MF electrode materials for energy storage devices, and it has a number of benefits. With this adaptable technique, metal oxide cathodes for LIBs can be produced from Fe oxides/hydroxides, FeF₃, M_xF_y, or M_xF_y/G composites. In addition, compared to other solution-based procedures, this method can withstand the rise in particle size brought on by Fe’s electronegativity, whose particles grow quickly after nucleation [37]. FeF₃ is converted from Fe₃O₄/G at a high rate, and the product yield is satisfactory. In a customised autoclave using a vapour–solid technique, the production of porous carbon materials on FeF₃ was effectively accomplished to create FeF₃/C nanocomposites. The results indicated that the autoclave was a critical element in driving the reaction that formed FeF3 nanomaterials by examining the phase changes that took place during the interaction of the HF solution with the precursors in an Ar environment. The charge capacities of the as-prepared FeF3/C nanocomposites are larger than those of the bare FeF₃ materials and demonstrate stable cyclic performance with a charge capacity of 200 mAh g⁻¹ [38]. They had charge capacities of 134.3, 103.2 and 71.0 mAh g⁻¹ at different current densities of 100, 500 and 1000 mA g⁻¹.

Current Challenges and Future Opportunities

Rechargeable Li batteries can benefit greatly from using metal fluorides as electrode materials, including greater safety and lower cost. When compared to conventional intercalation cathodes, metal fluoride cathode materials offer up to a 50% improvement in volumetric energy density and a twofold increase in cell-level specific energy.

Improving the conversion energy efficiency of metal fluorides by lowering the reaction overpotential is one of the biggest issues they face. The energy density of FeF₃ can reach 1341.7 and 1899.13 Wh kg⁻¹ when discharged or charged at 100 mA g⁻¹, respectively. In contrast to the results for transition metal oxides (95.8% for NMC and 93.9% for LiFePO₄), the conversion energy efficiency is only 70.7%, as shown in Fig. 14 [40]. Future study should place greater emphasis on the topological structure and the design of external wiring networks to address this issue.

Fig. 14

Based on their discharge–charge curves, energy efficiency and density were determined for A FeF₃, B LiFePO₄, and C transition metal oxide (NMC) [39]

To further optimise the spatial distribution of pristine phases, conversion products and conductive network components in electrodes, surface defect chemistry (metastable or framework phases) of metal fluorides should also be taken into account. The defect chemistry of the relevant phases, which includes stoichiometric changes and doping, is one of the main instruments. Instead of using the solid–solid conversion technique, liquid–solid conversion mechanisms may be used to increase the kinetics. It is worthwhile to look at conversion reactions involving Fe and LiBF₄ rather than LiF or to take boron-based additives into account to separate LiF [22]. To avoid future anode dendrite growth and cathode dissolving consequences, it is also required to investigate electrolytes additives, separators and binders.

Despite their promising characteristics, metal fluoride conversion cathode materials face several technological limitations that hinder their widespread industrial use. These challenges encompass substantial voltage hysteresis, low electrical conductivity, dissolution of active materials, volume changes during cycling, irreversible structural alterations and problematic interactions with electrolytes. The researchers have made noteworthy attempts to address these issues, rendering metal fluoride electrode materials viable for energy storage, the inherent complexities of the conversion reactions, involving intricate chemical pathways, highly insulating by-products like LiF, and interactions with transition metals, continue to pose challenges that affect cycling performance and voltage hysteresis. If these challenges can be fully addressed, metal/iron fluorides may indeed emerge as promising candidates as metal cathodes.

Conclusion

In conclusion, there are several strategies to improve metal fluoride cathodes, including: developing new desirable materials through the optimisation of nanostructures, which will reduce the paths for Li-ion diffusion and lead to high electrochemical performance. The needed nanoscale architectures such as metal oxides for an enhanced architecture of active metal fluoride materials may require a suitable synthesis method. In addition, it might aid in the construction of sophisticated cell component solutions, the invention or optimisation of the electrolytes employed and the acceleration of mass charge transport. It is obvious that a variety of techniques have been employed to enhance these electrode materials’ electrochemical performance to progress and perfect them; however, considerable advancements are still needed for the uses in practise.