Insights into the electrochemical performance of metal fluoride cathodes for lithium batteries

Low conductivity

Metal fluoride (FeF₃ and FeF₂)-based cathode materials are particularly promising materials due to their high volumetric and gravimetric capacities^[17]. However, the strong ionic bond between M and F results in a large band gap, which makes metal fluoride materials poor electronic conductors^[76,77]. The large interfacial energy of Fe/LiF enhances the mass transport resistance, which hinders the development of large metals and LiF clusters during the cycling process. Furthermore, the harsh interactions between the electrolyte and metal fluoride cathode additionally improve the cell resistance and contribute to Fe degradation and dissolution, thus leading to rapid capacity fading upon cycling and irreversible structural changes along with low rate capability^[19]. In addition, the slow separation of the Fe and LiF phases during the cycling process may lead to capacity fading and cell polarization growth^[27]. To overcome some of these problems, the development of various metal fluoride-based nanocomposites has been significantly explored^[78,79]. Various conductive carbons, including graphene, carbon blacks, carbon fibers, carbon nanotubes (CNTs), and micro- and mesoporous carbons, have been greatly explored in such hybrid composite cathode syntheses and exhibited noticeable improvements in battery performance^[17,80,81]. The poor electronic conduction of the electrode can be improved by the introduction of highly conductive materials, e.g., carbon and metal/metal oxides, which is effective in overcoming this limitation and positively improving the reversible capacity and stability of metal fluoride electrodes. The conductive species with an electron transport chain introduced into the active core material can maintain good electronic contact and effectively improve the kinetic behavior. Furthermore, the coating or adhesion of the external moieties on the active material surface efficiently hinders the side reaction and overpowers the dissolution.

Carbon-based metal fluoride nanocomposites

To improve the electrochemical performance and cycle stability, carbon-based nanocomposites have been used successfully because of their unique properties, such as high structural stability, high electronic conduction, large pore volume, and high surface area. Consequently, carbon composites with nanocomposites as core materials exhibit excellent electrochemical performance. Time, t, is proportional to the square of the diffusion length, L, during Li-ion diffusion, as shown in Equation (2)^[82-84]:

t = L²/D (2)

where D is the diffusion coefficient of Li ions.

The Badway group first reported carbon and metal fluoride nanocomposites (CMFNCs) as reversible cathodes to improve the electrochemical efficiency of metal fluorides^[85]. They synthesized the CMFNCs via a ball-milling method, which showed a reversible specific capacity of ~600 mAh g^-1 with a discharge voltage from 4.5 to 1.5 V. This result arises from the fact that the surface of nanosized crystals contains a number of surface defects that contribute significantly to improve the ionic and electronic activity. Nearly a third of the discharge capacity progressed in a cathode reduction reaction of Fe³⁺ to Fe²⁺ between 3.5 and 2.8 V, while the remaining specific capacity was provided by a two-phase conversion reaction at ~2 V, forming a finer Fe/LiF nanocomposite. Furthermore, the preliminary results of CoF₂, NiF₂, and FeF₂ CMFNCs were also used for comparison in the discussion of the electrochemical performance of the metal fluoride-based conversion reaction.

Despite intensive efforts, the poor cycling stability and partial reaction irreversibility have largely hampered the application of metal fluoride cathodes^{[18,19,86-88]}. Therefore, an alternative method for the synthesis of cathode materials is desirable. For this reason, the intercalation of metal fluorides into a carbon matrix might be an effective approach to increasing the cycling performance. In particular, metal oxide-encapsulated carbon electrode materials have demonstrated superior cycling performances in LIBs^[89]. Based on these concepts, the incorporation of metal fluoride into carbon materials, such as graphene, carbon fibers, CNTs, and mesoporous carbon materials, may lead to enhanced electrochemical properties. Therefore, Prakash et al. demonstrated ferrocene-based carbon-iron and LiF nanocomposite by pyrolysis of a mixture of LiF and ferrocene at 700 °C under an Ar atmosphere^[90]. The composite was comprised of onion-type graphite structures and multi-walled CNTs, in which the LiF is dispersed within the carbon matrix and Fe₃C and Fe nanoparticles are incorporated in the carbon matrix [Figure 5A and B]. The nanocomposite presents a specific surface area of 82 m² g^-1 and consists of both meso- (0.14 cm³ g^-1) and micropores (0.025 cm³ g^-1). The as-prepared cathode reached a reversible specific capacity of ~300 mAh g^–1 at a current density of 20.83 mA g^-1 with a potential range from 0.5 to 4.3 V. It showed good capacity retention (83.5% over 200 cycles) and excellent rate capability [Figure 5C and D]. Hence, due to its moderate capacity, more work is desired to optimize its reversible capacity, such as producing a uniform particle size/distribution, removing/reducing Fe₃C, reducing the carbon content, obtaining more Fe-encapsulated CNTs, and so on. Similarly, another group fabricated FeF₃ nanoflowers grown on CNT branch (FNCB)-based nanoarchitecture cathode materials by the functionalization of CNT surfaces^[76]. The FNCBs exhibited improved electron transport and Li-ion storage when employed as cathode materials.

Figure 5. (A) TEM images of nanocomposite electrode after charging experiment. (B) Mossbauer spectrum of nanocomposite at 300 K. (C) Specific capacity and cycle performance of cathode. (D) Rate performance of as-prepared samples. Reproduced from Ref.^[90] with permission. Copyright 2010 Royal Society of Chemistry.

Graphene and reduced graphene oxide (RGO) stand out among carbon-based materials due to their outstanding mechanical properties and electrical conductivity^[91-95]. Therefore, graphene sheets may be preferred conductive networks and building units to deliver facile electron pathways. A vapor-solid method was used for the first time to fabricate graphene-wrapped FeF₃ nanocrystals (FeF₃/G) as cathode materials [Figure 6A]^[96]. Compared to bare FeF₃, the as-prepared FeF₃/G nanocrystals supply an improved capacity of 155 mAh g^-1 at a current density of 104 mA g^-1, as well as exhibiting superior rate capability and cyclic stability over 100 cycles, which could be attributed to the lower electrical resistance and buffering effect of the graphene layers [Figure 6B and C]. Liu et al. prepared an iron fluoride-reduced graphene nanocomposite cathode by a cost-effective low-temperature solution phase method^[97]. Notably, the resulting iron fluoride-graphene nanocomposite cathode exhibited a high specific capacity of 210 mAh g^-1 at 0.2C and demonstrated superior rate capability at high current densities (1, 2 and 5C).

Figure 6. (A) Schematic illustration of fabrication procedure of FeF₃/G nanocomposite. (B) Rate capability and (inset) voltage profiles of FeF₃/G and bare FeF₃. (C) Cycling performance and Coulombic efficiency of FeF₃/G nanocomposite. Reproduced from Ref.^[96] with permission. Copyright 2010 Royal Society of Chemistry.

Similarly, Qiu et al. reported the seeding of FeF₃·0.33H₂O nanoparticles on RGO via an in situ approach and achieved enhanced cycling stability and high particle loading, attributed to the chemical tuning of the interfacial bonding between rGO and FeF₃·0.33H₂O [Figure 7A and B]^[98]. Specifically, the FeF₃·0.33H₂O/rGO nanocomposites exhibit a higher discharge capacity of ~208.3 mAh g^-1 at a current density of 0.5C and excellent cycle stability (133.1 mAh g^-1 after 100 cycles at 100 mA g^-1) with 97% capacity retention [Figure 7C and D]. Later, highly uniform carbon-based nanocomposites with nanoconfined FeF₂ were successfully fabricated via a vacuum impregnation technique by the intercalation of a fluoride precursor with activated carbon powder^[15]. Carbon pore walls prevent the physical separation of fluoride particles, accommodate fluoride volume changes during lithiation/delithiation, and supply holes or electrons to the electrochemical reaction sites during cell operation. When metal fluoride nanoparticles are confined into carbon nanopores, the nanocomposite displays dramatically superior electrochemical performance and cyclic stability in LIBs.

Figure 7. (A) Schematic illustration of preparation processes for FeF₃·0.33H₂O/rGO. (B) SEM and TEM images of FeF₃·0.33H₂O/rGO. (C) Cycling performance of FeF₃·0.33H₂O/rGO and FeF₃·0.33H₂O without rGO at a current density of 500 mA g^-1. (D) Rate performance of FeF₃·0.33H₂O/rGO and FeF₃·0.33H₂O NPs. Reproduced from Ref.^[98] with permission. Copyright 2018 American Chemical Society.

Previous research studies concentrated on introducing FeF₃ particles to the surface of carbon matrices or infusing them inside the nanopores of RGO, CNTs, and mesoporous carbon^[30,97,99]. However, the synthesis of carbon matrices requires complex synthetic methods and is costly. Moreover, successfully creating the intimate contact between carbon matrices and FeF₃ nanoparticles is not easy, so avoiding the larger fraction of FeF₃ outside the inner pores is very challenging^[100]. Nevertheless, a FeF₃-carbon composite with intimate contact between the conductive carbon matrices and FeF₃ nanoparticles has been fabricated by modified carbonization and polymerization processes. This method can effectively enhance the intimate contact between carbon and FeF₃ by the homogeneous mixing of FeCl₃ (FeF₃ precursors) with organic precursors (which can be converted into conductive carbon) at the molecular level. Thus, composites of FeF₃ nanoparticles wrapped in graphitized carbon were prepared via a facile polymerization process using citric acid (C₆H₈O₇), FeCl₃, and ethylene glycol as a carbon source, iron precursor, and crosslinking agent, respectively^[101]. The as-synthesized graphitic carbon-coated FeF₃ composite displayed a capacity of 188 mA h g^-1 at initial discharge. After slow activation processes, the capacity finally reached 421 mAh g^-1 in the voltage range from 1.5 to 4.5 V, which is superior compared to bare FeF₃. The enhanced cycling and rate performance of this nanocomposite were mainly accredited to the controlled size of the FeF₃ particles and the conductive graphitic carbon coated on the surface of FeF₃ during prolonged cycles.

In comparison to inorganic materials, various organic compounds have also been considered electrode materials but have received less attention due to the development of conductive polymers (CPs) and the achievement of intercalation compounds (e.g., LiCoO₂) in the 1980s as cathode materials for LIBs^[102,103]. However, the intrinsic limits of intercalation compounds as cathodes for LIBs now desire innovation through the use of organic electrodes^[104]. Therefore, carbonyl compounds are generally superior to CPs as electrode materials and are remarkable candidates for building low-cost, sustainable and functional energy storage systems^[105]. For this reason, a hybrid FeF₃@Li₂C₆O₆/rGO nanocomposite was prepared by morphology control, intercalating RGO with spherical Li₂C₆O₆, followed by modification with a FeF₃ coating^[106]. The as-prepared FeF₃@Li₂C₆O₆/rGO hybrid cathode displayed a superior cycling performance (320 mAh g^-1 after 100 cycles), attributed to the coating of electrochemically active FeF₃ on the surface of Li₂C₆O₆, which successfully inhibited Li₂C₆O₆ dissolution in the electrolyte. This interface engineering approach and morphology control represent a feasible and scalable technique to effectively increase the rate performance and cycling properties of organic-based cathode materials. Recently, Reddy et al. synthesized carbon-FeF₂ nanocomposites by a facile one-step method at 250 °C, combining fluorinated carbon (CF_x) with iron pentacarbonyl -[Fe(CO)₅]^[107]. The resulting C-FeF₂ nanocomposites act as the cathode and achieve improved electrochemical properties. By reacting Fe(CO)₅ with four different CF_x precursors, they synthesized four different C-FeF₂ nanocomposites, which were made of carbon black, petcoke, carbon fibers, and graphite. The excellent electrochemical performance was also evaluated at 25 °C and 40 °C for high energy density LIBs.

Fu et al. reported an effective and versatile strategy for the development of metal fluoride-carbon nanofiber nanocomposites as flexible, free-standing cathodes^[108]. The as-synthesized assembled FeF₃-C/Li cells exhibit a higher discharge capacity of 550 mAh g^-1 at 100 mA g^-1 and demonstrated excellent stability (> 400 cycles with no structural degradation). These promising characteristics can be effectively attributed to the robustness of the electrically conductive carbon network, the nanoconfinement of FeF₃ nanoparticles, reducing the irreversible separation and aggregation, the ultrafast pathways for electron transport and ion diffusion, and the inhibition of undesirable reactions between the liquid electrolyte and active materials.

Another effective and common method to enhance the electrochemical properties is the composite design of active materials. The first choice is always carbon-based metal fluoride composite electrodes. For instance, FeF₂ nanoparticles confined into carbon nanofibers can suppress the side reactions between FeF₂ and the electrolyte, thereby affording an enhanced electronic conductivity in the electrode and cycle stability over 400 cycles^[108]. According to previously reported studies on battery electrode materials, the composite architecture design is a very capable approach to avoiding interactions between the active materials and electrolytes and accomplish fast transport in the electrode^[109]. However, thus far, it is still a challenge to encapsulate nanosized FeF₂, FeF₃, CoF₂, and CuF₂ into a three-dimensional (3D) carbon matrix for the self-protection of electrodes and achieving fast electron and Li-ion transport. Simultaneously, the 3D architecture design can offer cathodes a high-active mass loading, high-rate performance, and good cycling stability. Therefore, Wu et al. prepared a FeF₃@C composite with a 3D honeycomb architecture by a simple and facile method^[110]. The isolated FeF₃ particles with sizes of 10-50 nm were uniformly distributed in the 3D carbon honeycomb carbon framework, where the honeycomb carbon walls and hexagonal-like channels (cells) deliver sufficient conduction pathways for achieving fast ion diffusion and electron transport in FeF₃ cathodes. According to the results of the report, the as-prepared 3D honeycomb architecture FeF₃@C composite cathodes with high areal FeF₃ loadings (2.2 to 5.3 mg cm^-2) demonstrated an unprecedented rate capability of up to 100C and remarkable cycle stability within 1000 cycles, with high capacity retentions (95%-100%) within 200 cycles at 2C. As a result, it demonstrated that the 3D architecture with a honeycomb morphology represents a powerful strategy of composite design for metal fluorides in order to obtain exceptional electrochemical performance in metal fluoride Li batteries.

Overall, the results reported up to now display excellent potential for optimizing the architecture and composition of carbon-based metal fluoride cathodes to enhance their rate performance and cycling stability. Nevertheless, improving the electrochemical performance of carbon-based metal fluoride cathodes to the level of commercial applications still requires additional developments in novel fabrication techniques capable of precisely controlling the overall morphology and the nanoscale features of the carbon matrix and active materials. Therefore, more efforts are required to remove these issues for their commercialization.

Metal/metal oxide-based fluoride nanocomposites

Recently, the use of doping has been explored widely for intercalation-type electrode materials^[111]. For conversion-type electrode materials, doping may also deliver obvious benefits to improve the ionic and electronic conductivities of electrode materials by enhancing the cathode performance through the intercalation of heterogeneous/homogenous metals, vacancies, or ions. Furthermore, many scientists have highlighted the possibility of decreasing the cluster size of creating ‘‘true’’ conversion electrode materials upon the lithiation process, attributed to the reduction in the interfacial energy and modification of the ion mobility of the clusters. As a result, this may improve the energy efficiency and cyclic stability of such electrodes because the affinity of the slow and steady separation of the cluster size could be diminished and the voltage hysteresis for the charge/discharge could be mitigated by reducing the paths for mass transport. In several recent reports, doping with Si has significantly enhanced the electronic conductivities of the electrode materials, with remarkable discharge capacity utilization obtained as a result^[112].

Molybdenum bisulfide was ball milled with FeF₃ to improve its electrochemical properties^[113]. The orthorhombic structured FeF₃/MoS₂ with a uniform morphology showed excellent electrochemical performance as a cathode material in LIBs. The initial discharge capacity of the material was 170 mAh g^-1 at a 0.1C rate with a voltage range from 2.0 to 4.5 V, and it maintained a specific capacity of 83.1% after 30 cycles. FeF₃ with an orthorhombic structure was prepared by Wu et al.^[87] via a liquid phase method using HF, FeCl₃, and NaOH as precursors. The obtained product was further ball milled with conductive V₂O₅ powder to obtain a FeF₃/V₂O₅ nanocomposite. The electrochemical properties of the prepared material showed a significant improvement after the addition of V₂O₅. The FeF₃/V₂O₅ nanocomposite exhibited good rate and cycle performance. A discharge capacity of 209 mAh g^-1 was obtained at a retention rate of 0.1C and a voltage range of 2.0-4.5 V and was maintained after 30 cycles.

A TiO₂-coated FeF₃·0.33H₂O cathode material with a spherical morphology was prepared via a solvothermal method by Zhang et al.^[114]. The nanosized FeF₃·0.33H₂O was coated with uniform TiO₂ with an average particle size of ~1.0 μm and good dispersion ability. The initial reversible capacity at the retention rate of 0.1C was 654 (discharging) and 522 (charging) mAh g^-1, respectively, with a voltage range of 1.5-4.5 V. After 200 cycles, a good stability of 264 mAh g^-1 was maintained by the nanomaterial cathode, which demonstrated that the TiO₂ layer on iron fluoride could be a promising cathode material for LIBs. A novel core-shell FeF₃@Fe₂O₃ nanocomposite (100-150 nm) and tunable iron oxide (Fe₂O₃) were prepared via a simple heat treatment using fine structured FeF₃ as a precursor^[115]. The as-synthesized FeF₃ and core-shell FeF₃@Fe₂O₃ nanocomposites were characterized and their electrochemical performance was studied. A comparison between the pristine FeF₃ and the FeF₃@Fe₂O₃ nanocomposite revealed a significant improvement in the electrochemical performance upon the in situ coating of Fe₂O₃, even when the coating amount was 0.6-5.2 wt.%.

Metal doping has a positive effect on the charge/discharge capacity of metal fluoride cathode materials. The doped metal can increase the Li-ion diffusion coefficient and improve the electrochemical performance of the electrode materials. Recently, copper-doped FeF₃ nanomaterials have been synthesized via a liquid-phase method^[116]. The electrochemical reversibility of Fe_1-xCo_xF₃ (x = 0, 0.03, 0.05 or 0.07) materials was further enhanced by the addition of acetylene black via mechanical a ball-milling process to obtain Fe_1-xCoxF₃/C nanocomposites. It showed a significant improvement in electrochemical performance with the introduction of cationic Co. The discharge capacities at retention rates of 1, 2, and 5C were 150, 140, and 125 mA h g^-1 and maintained the capacity after 100 cycles as high as 92.0%, 92.2%, and 91.7%, respectively. Metal fluoride cathode materials can suffer from particle fracture during lithiation/delithiation in LIBs. To improve the cathode nanoparticle stability and overcome this phenomenon, Co doping has been carried out for FeF₃ in LIBs by Zhang et al.^[117]. In this study, calculations for Co doping onto the hexagonal tungsten bronze structure were carried out successfully to form Co_xFe_1-xF₃ systems (x = 0.08, 0.17 or 0.25). The study showed a sharp decrease in the band after Co doping, which may be attributed to the presence of Co 3d impurity energy levels between the conduction and valence bands. A thermally stable Co-doped iron fluoride (Fe_0.9Co_0.1F₃·0.5H₂O) was synthesized by a non-aqueous precipitation method as a cathode material for LIBs^[118]. The thermogravimetric analysis shows that the cathode nanomaterial was stable until 243 °C, with the crystal structure collapsing beyond this temperature due to water content elimination. The Co-doped cathode material showed a high discharge capacity (227 mAh g^-1) during cell cycling and retained a reversible capacity of 150 mAh g^-1 after 200 cycles.

A significant boost in the electrochemical performance of FeF₂ cathode materials was found after the addition of NiF₂ nanomaterials for LIBs by Huang et al.^[119]. The data obtained revealed the precise control of morphology and composition of the desired FeF₂-NiF₂ nanoparticles and undesirably enhanced capacity fading of the electrochemical cell. In another study, the substitution of Ni with Cu was carried out for binary NiF₂ nanomaterials and their electrochemical performance was studied for LIBs^[120]. From in situ TEM, the structure of different ternary metal fluorides with Cu substitution was observed and the results revealed that when the Cu substitution was from 1-25 wt.%, the areal expansion reduced during the first lithiation. Furthermore, due to the reversible reaction, the fluorine loss during the delithiation was also reduced, which proved Cu to be a better choice for substitution to improve the electrochemical performance. Recently, ternary metal fluorides, like AgCuF₃ and CuxFe_1-xF₂, have been used as cathode materials in LIBs by Li et al.^[121]. Thin-film Cu-Fe-F (CFF) cathode materials were successfully grown in situ via a pulsed laser deposition method. The as-prepared CFF cathode material showed a high specific capacity of 420 mAh g^-1 and good cyclic stability even after 100 cycles, which may be attributed to reversible structural rearrangement after delithiation powered by high-resolution TEM, in situ XPS, and selected electron diffraction techniques.

Voltage hysteresis

Hysteresis is a common problem between the charge and discharge potentials in rechargeable batteries, which is related to thermodynamic and kinetic factors^[9,10,18]. It may reduce capacity utilization and leads to low energy efficiency. In addition, the charge and discharge window with a larger voltage hysteresis may introduce more side reactions between the active substance and electrolyte and possibly unstable SEI. Unfortunately, MF cathodes show undesirably large voltage hysteresis, which is typically related to the poor electronic conductivity of the materials, rapid degradation of the battery, and changes in surface/interfacial energies during the conversion reactions. Energy losses for compensating the activation energies of breaking chemical bonds are also another key reason for voltage hysteresis. The discharge process (lithiation) of MF cathodes involves breaking Fe-F bonds and forming lower free-energy compounds containing lithium. The products from discharge process, such as LiF (bonding energy of Li-F is 577 kJ mol^-1), are thermodynamically stable due to the large electronegativity difference between Li and F. The charge process (reverse reaction) is required to decompose LiF by breaking the stronger corresponding chemical bonds. This introduces a large activation energy barrier, which contributes to a large voltage hysteresis (overpotential). This large voltage hysteresis even leads to the misapprehension that cells based on conversion MF cathodes are only primary batteries, such as Li-CuF₂ battery systems^[50,55,60].

Recently, Li et al.^[48] employed in situ analytical techniques to correlate the voltage profile with intermediate phases of FeF₃, which involved the evolution and spatial distribution of intermediate phases during the discharge-charge process. The results show that the phase evolution in the electrode is symmetric during cycling. However, the spatial evolution of the electrochemically active phase controlled by the reaction kinetics is different. They found that the kinetics of the FeF₃ electrode in nature is the reason for the voltage hysteresis. It originates from the Ohmic voltage drop, overpotential, and different spatial distributions of active phases, as shown in Figure 8. Therefore, the large hysteresis can be alleviated through the reasonable optimization of the material and electrode microstructure.

Figure 8. Schematic illustration of phase evolution in the electrode during discharge and charge. Reproduced from Ref.^[48] with permission. Copyright 2016 American Chemical Society.

Elemental doping is a promising approach for conversion-type materials to overcome the problem of voltage hysteresis. The ionic and electronic conductivities of the electrode material are enhanced by the introduction of homogeneous/heterogeneous metallic (non-metallic) ions or vacancies. For example, FeF₂ doped with Cu and FeF₃ doped with Co showed lower voltage hysteresis. The partial substitution of F with O in FeF₃ greatly improves the reaction kinetics, reduces voltage hysteresis, and exhibits excellent cycle performance over 100 cycles (for this chemistry). Recently, Wang et al. proposed a novel strategy to make full use of the Cu²⁺/Cu⁰ redox range for the first time in the application of copper-based fluorine rechargeable Li batteries^[122]. The prepared FeF₂ (tetragonal rutile) and CuF₂ (monoclinal distorted rutile) solid solutions gave Cu_1-xFexF₂ 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, as shown in Figure 9. Cu_0.5Fe_0.5F₂ displays a reversible capacity of 543 mAh g^-1 at the first discharge-charge process. As-prepared cathode materials show enhanced dynamic performance, which lies in a reduction of the voltage hysteresis and the elimination of the voltage drop observed in FeF₂. After first-half conversion, the lattice disorder in FeF₂ increases due to the down-sizing of FeF₂, which is responsible for the disappearance of voltage drop and higher discharge voltage during the initial second-half conversion. There is a small voltage hysteresis of Cu²⁺/Cu⁰ due to the low nucleation barrier for the formation/decomposition of the Cu-F bond.

Figure 9. Illustration of potential reaction pathway of Cu_1-xFe_xF₂ during discharge-charge. During discharge (stages I and II), Cu and Fe occur sequentially by the conversion reduction. During charge, the oxidation processes of Fe and Cu (stages III and IV) overlap to reform the disordered rutile-like Cu-Fe-F phase. Reproduced from Ref.^[122] with permission. Copyright 2015 The Author(s).

Side reactions with electrolytes

The unfavorable side reactions between electrodes and electrolytes increase voltage hysteresis, reduce the Coulombic efficiency and stability of the cathode SEI and induce a safety hazard. There are multiple roles for the SEI, including limiting side reactions between the cathode (or anode) and the electrolyte. An ideal SEI layer has properties that include high cation conductance and electrical resistance, a thickness of a few nanometers, high mechanical toughness, and stability over a wide range of voltages. A stable SEI is an accepted prerequisite for safe battery performance, be it with a metal or an ion insertion anode. In theory, conversion-type cathodes should be advantageous in the thermodynamic stability of electrolytes because of their low electrochemical potentials (1.5-4.0 V vs. Li/Li⁺). However, the oxidation stability of electrolytes can be changed by various salt anions, especially in concentrated electrolytes, and certain cathode chemicals. In addition, since the conversion cathode is usually discharged to a relatively low potential vs. Li/Li⁺ (sometimes down to 1.2 V), the electrolyte may be reduced at the cathode surface. At moderate cathode potentials, the various substances formed by electrolyte reduction are more easily oxidized than the original electrolyte. Furthermore, some conversion cathode species may also catalyze the reduction reaction. For example, it has been found that the cyclic stability of MFs is worsened by the formation of a lithium carbonate species, which is susceptible to nanometal catalyzed reduction during the battery discharge to 1.2-2.0 V. The continuous large volume changes during cycling may prevent stabilization of the cathode SEI with poor elasticity in conversion-type cathodes. However, in contrast, the formation of a stable cathode SEI may also contribute to the stabilization of the conversion cathode, preventing adverse reactions between the cathode and electrolyte.

In the case of liquid electrolytes, modifications have been made to the composition of lithium salts, organic solvents, salt concentrations, and organic and inorganic electrolyte additives in an attempt to alleviate the limitations of cathodes based on the conversion reaction. For intercalated cathodes and graphite anodes, classic organic carbonate esters are commonly used solvents, such as ethyl carbonate, propylene carbonate, ethyl carbonate, and ethyl carbonate. However, for the partial conversion of the cathode, the carbonate solvents may cause some adverse reactions. To address this problem, an electrolyte with a high concentration of LiTFSI salt has recently been developed to inhibit the dissolution of cathode materials by in situ forming a protective layer penetration on the surface of the electrode. In contrast, an ultrathin artificial SEI layer was prepared by atomic layer deposition, and it was reported that the conformation of the layer covered with fluorine particles reduced the side reaction between fluorine and the electrolyte. For example, 3D Al₂O₃-coated FeF₂ nanoparticles were prepared on Ni support by Kim et al.^[83]. The results revealed that the electrochemical applications of the as-prepared electrode were significantly improved due to the ion transport pathway. The initial discharge capacity of the 3D FeF₂ electrode was 380 mAh g^-1 at 200 mA g^-1. The reaction kinetics and specific capacity improved due to the 3D support, which makes an effective platform for electron transfer and shortening the Li-ion diffusion pathway. Therefore, more attention should be paid to the SEI to significantly improve the cyclic stability of metal fluorides.

Hydrothermal methods

Hydrothermal methods have often been used in synthetic chemistry due to the advantages of the crystalline powder at high temperatures. The controlled reaction conditions of the hydrothermal process result in the desired morphology^[123-125]. In this process, the product prepared has good dispersion, high purity, uniform morphology and controlled particle size. It can be used to synthesize monocrystalline materials, as well as 2D and 3D materials, with particle sizes ranging from microns to nanometers.

The low electronic conductivity of FeF₂ cathode nanomaterials for LIBs was enhanced via the incorporation of ordered mesoporous carbon (CMK-3)^[126]. The FeF₃ precursor was synthesized with the hydrothermal treatment of CMK-3 and iron oxide, followed by a topochemical process. The FeF₃ precursor was further annealed to obtain a hierarchical conductive FeF₂/CMK-3 nanoparticle network, as shown in Figure 10A. This type of hierarchical structure offers continuous electronic conduction within the network and porosity for the volume expansion of the prepared material. The prepared cathode material showed a stable cycle life of 1000 cycles with 0.3% capacity loss per cycle. After 100 cycles, high discharge capacities at current densities of 500, 2000, and 4000 mA g^-1 were 500, 400, and 320 mAhg^-1, respectively, as shown in Figure 10B.

Figure 10. (A) Schematic illustration of the synthesis of porous FeF₂-CMK-3 composite. (B) Electrochemical performance of FeF₂/CMK-3 cathodes: (a) charge-discharge profiles at 200 mAg^-1; (b) and (c) cycling performance. Reproduced from Ref.^[126] with permission. Copyright 2015 Royal Society of Chemistry.

Solvothermal methods

Solvothermal methods are actually developed based on hydrothermal methods, except that the reaction medium is organic solvent rather than water. In a solvothermal process, one or more precursors in the reaction are dissolved under critical circumstances. This process has numerous advantages, including short reaction times, fast reaction kinetics, uniform particle distribution and high crystallinity. The electrode materials obtained by solvothermal methods have shown enhanced electrochemical performance, including good rate performance and long cycle life^[127-129]. Recently, anhydrous CuF₂ nanomaterials from alkoxides Cu(OR)₂ (R = Me or tBu) were synthesized in hydrofluoric acid and tetrahydrofuran as reaction media^[130]. A schematic diagram of the process is shown in Figure 11. Depending on the reaction conditions, different sizes of 10 and 100 nm nanoparticles were obtained. The product was found to be very hygroscopic and formed various hydrated products, including Cu(OH)F, CuF₂·2H₂O, and Cu₂(OH)F₃. The as-prepared nanomaterials showed high electrochemical performance at a potential ~2.7 V for Li-ion charge/discharge. Different specific capacities were obtained with different particle sizes, e.g., 468 and 353 mAh g^-1 for ~8 and ~12 nm crystalline diameters, respectively, for CuF₂. The high specific capacity of the nanomaterials faded after several cycles and the cell voltage decreased to 2.0 V, which was attributed to the irreversible nature of the reacting material involved in the electrochemical cell reaction.

Figure 11. Overview of synthesis reactions. Thick red arrows represent most suitable reactions toward CuF₂. Dashed arrows represent product mixtures. Green boxes represent compounds used for electrochemical investigations. Reproduced from Ref.^[130] with permission. Copyright 2018 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim.

In another study, Fe_(1-x)Co_xF₃/multi-walled carbon nanotube (MWCNT) nanocomposites were synthesized by a solvothermal method^[131]. The iron fluoride nanomaterials doped with Co were wrapped with MWCNTs. The results of the prepared materials revealed the crystal structure adjustment, decreased the band gap, and enhanced the Li-ion diffusion capacity after Co doping. Moreover, the MWCNT wrapping increased the conductivity, which in turn enhanced the electrochemical performance of the Fe_0.96Co_0.04F₃/MWCNT nanocomposites. The high specific capacity of the nanocomposites at 2.0-4.5 V recorded was 217.0 mAh g^-1 at a rate of 0.2 C. The electrochemical performance was higher compared to the FeF₃/MWCNT nanocomposite counterparts. After 50 cycles, the specific capacity of the two nanocomposites decreased to 187.9 and 160.7 mAh g^-1, respectively, which showed the cyclic stability of the prepared nanocomposites for Li storage in Li batteries.

Microwave-assisted methods

For the synthesis of metal fluoride nanomaterials, microwave-assisted approaches are promising routes compared to other synthetic methods. Since its first report in the field of synthetic chemistry and materials synthesis, this method has grown rapidly in this area of research^[132]. By this method, the reaction entirely depends on the rapid heating, which produces reagents and solvents and makes this different from other methods, e.g., hydrothermal and solvothermal methods. Furthermore, the fast heated reaction results in a high reaction rate and efficiency and lower energy consumption^[133]. In recent years, to obtain the tailorable morphologies, a combination of room-temperature ILs and a microwave heating process have been used^[134]. A microwave-assisted approach was followed for the successful synthesis of self-assembled iron fluorides (HMIFs) with a hierarchical mesoporous structure^[39]. The detailed possible steps and mechanism are shown in Figure 12. The dual nature fluorides, consisting of FeF₃·H₂O and Fe_1.9F_4.75·0.95H₂O, were built from a large number of nanorods with more than a dozen nanometer size and their electrochemical performance in LIBs investigated. The as-prepared HMIFs exhibited a homogeneous morphology with a hierarchical mesoporous structure and partial internal hollow core and possessed a specific area of 94 m² g^-1. By employing any conducting agent, the nanomaterials showed a high reversible capacity of 200 mA h g^-1 and excellent cycle stability with a specific capacity of 148 mA h g^-1 at 0.1C after 100 cycles. Moreover, the specific charge capacity exceeded 100 mA h g^-1 at 5C, which showed the advancements of HMIF materials for high electrochemical performance applications in LIBs. This extraordinary performance was attributed to the formation of a unique structure that provided large storage sites for Li, as well as a large contact area for the electrode/electrolyte and hence a volume change during lithiation/delithiation and the facile mass transfer of electrons and Li ions during the electrochemical cell reaction.

Figure 12. Schematic diagram of formation mechanism of partially hollow HMIFs. From steps 1 to 5: mixing the reactants; combining the solvated ions; initial stage emerging of nanoparticles; Ostwald ripening process; final state of the products. Reproduced from Ref.^[39] with permission. Copyright 2014 Royal Society of Chemistry.

Nanosized NaFeF₃ perovskite particles were synthesized from sodium ethoxide, ammonium fluoride and pre-synthesized FeF₂ rutile colloidal particles via a microwave-assisted method^[135]. A schematic diagram, powder X-ray diffraction phases, which confirmed the expected crystal structure of rutile and orthorhombic-type crystal, and the 3D structure models are shown in Figure 13. The as-prepared perovskite NaFeF₃ nanomaterials showed excellent electrochemical performance with low polarization for Na and Li storage in energy storage devices. This enhanced performance of excellent capacity retention is attributed to the stable corner-sharing cubic Li and Na perovskite with stable cycle and low volume change and thermodynamic cost, as supported by a polymorphism theoretical study.

Figure 13. (A) Synthetic process of NaFeF₃ electrode material. (B) XRD patterns of as-prepared FeF₂ and NaFeF₃. (C) 3D structural models from XRD experiments. Reproduced from Ref.^[135] with permission. Copyright 2019 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim

Sol-gel methods

Sol-gel methods are versatile routes in synthetic chemistry for designing complex nanostructures with controlled particle size, the arrangement of nanopores, and uniform particle distributions. The nanoporous behavior of the particle has the property of being functionalized during the gel formation even in a dry state by polymers or other active organic molecules. Upon careful heat treatment and utilization of the reaction precursor solution with temperature control, nanoparticles can be grown within the nanopores, which can be used in different applications with excellent performance^[136]. There are some unique advantages of the sol-gel method, which make this process special compared to other conventional methods used for nanomaterial preparation. The first is the formation of a viscous solution from the dispersion of the raw materials during the process. A homogeneous mixture is achieved in a short period and even after the gel is formed, a molecular level homogeneous solution of the reactants is obtained. The second is that during the reaction, it is easy to add other materials, so molecular-level homogeneous doping is possible. The third is that the reaction can be carried out even at low temperatures and easily compared to the reaction involving solid components. This is because, in the sol-gel method, the reactants diffuse in the nanometer range, while for solid particles, the diffusion is at the micro level, and as a result, low temperature is required for the nanoscale formation.

FeF₃ nanocrystals as electrodes for LIBs with a particle size of 30 nm were prepared by a sol-gel method^[137]. To enhance the electrochemical performance, the as-synthesized particles were fabricated with RGO, which shows a high specific retention of 150 mA h g^-1 and stable cycle life for Li ions, even after 50 cycles. In another study, LiF- and FeF₂-based nanocomposites were prepared via a sol-gel method in ethanol for LIBs^[138]. The SEM and TEM characterization of the LiF/FeF₂ composite proved that ~10 nm nanosized LiF and FeF₂ crystals were obtained, as shown in Figure 14A, with EDX and XRD analysis and a large surface area of 119 m² g^-1. The large surface and adsorption-desorption hysteresis revealed the presence of mesopores, which might have a positive impact on the electrochemical performance. The initial discharge specific capacity of the reversible conversion reaction for the nanocomposites recorded was 225 mAh g^-1 at a current rate of 10 mA g^-1 with a stable life cycle, as shown in Figure 14B.

Figure 14. (A) (a) and (b) SEM images of LiF-FeF₂ composite. EDX mappings of (c) Fe and (d) F. (e) and (f) TEM images of LiF-FeF₂ electrode. (B) Charge-discharge performance: (a) charge-discharge profiles at current density of 10 mA g^-1; (b) cycling performance for first 20 cycles at 10 mA g^-1; (c) rate capability (from 10 to 1000 mA g^-1). Reproduced from Ref.^[138] with permission. Copyright 2019 Elsevier B.V. All rights reserved.

Ionothermal methods

Ionic liquids (ILs) have been discovered as alternatives to conventional solvents^[139]. ILs are composed of cations and anions, in which one of them should be naturally organic and have a melting point below some arbitrary temperature^[140]. This class of solvent is divided into two major groups based on their melting points, namely, room-temperature ILs, which melt at room temperature, and near room-temperature ILs, which have a melting point below 100 °C. Low vapor pressure is the distinguishing feature that makes them different from other solvents^[141]. ILs were categorized as convenient alternatives to organic solvents much later, although they were recognized as solvents at the beginning of the 20th century. The main characteristics of ILs include good electric conductivity, fire resistance, high thermal stability, ability to dissolve many inorganic and organic compounds, nonvolatile, a wide range of temperatures, and recyclability^[142,143]. These characteristics made ILs a good choice for energy storage devices. The in situ synthesis of FeF₃/GNS nanosheet hybrid nanomaterials was developed via an IL-assisted method^[144]. The role of ILs was not only as green fluoride sources but also to provide uniform dispersion and tight surface modification of FeF₃·0.33H₂O on graphene nanosheets. The synthesis procedure of FeF₃·0.33H₂O/GNS and the discharge profile at different currents are shown in Figure 15, where it can be seen that the as-synthesized FeF₃·0.33H₂O/GNS cathode nanomaterials showed an extensive enhancement in both specific capacity and rate performance due to the electron transfer path of the iron nanoparticles and GNS in LIBs. The cyclic stability proves the strong adhesion of iron fluoride nanoparticles, which have a significant electrochemical performance of 115 mAh g^-1 after 250 cycles, even at 10C, which is attributed to the strong structure of the hybrid and robust interaction between graphene and iron fluoride nanoparticles. In another study, mesoporous FeF₃·0.33H₂O cathode materials for LIBs were prepared at low temperatures via ILs as reaction media^[23]. The enhanced electrochemical performance in terms of high reversible capacity and reactive voltage of the carbon free FeF₃·0.33H₂O cathode materials in LIBs was expected due to the morphology and optimization of hydration of the water-induced microstructure at room temperature.

Figure 15. (A) Synthetic process of FeF₃·0.33H₂O/GNS composite. (B) Discharge profiles and (C) cycling performance of FeF₃·0.33H₂O/GNS composite and FeF₃·0.33H₂O without GNS. Reproduced from Ref.^[144] with permission. Copyright 2013 American Chemical Society.

Vapor-solid methods

A vapor-solid method was used to thermally convert Fe₃O₄/graphene into FeF₃/G cathode materials for LIBs^[145]. This method has several advantages and has been used to prepare MF electrode materials for energy storage devices. This versatile method can be applied to prepare FeF₃ and metal oxide cathodes from Fe oxides/hydroxides and can be used to fabricate M_xF_y or M_xF_y/G composites for LIBs. Furthermore, this process can resist the increase in particle size due to the electronegativity of Fe, whose particles grow rapidly after nucleation compared to other solution-based processes. The conversion rate of FeF₃ from Fe₃O₄/G is also high and the product yield is good. The fabrication of porous carbon materials was successfully achieved on FeF₃ to prepare FeF₃/C nanocomposites in a tailored autoclave via a vapor-solid method^[99]. The phase changes during the reaction between the HF solution and precursors in an Ar atmosphere were examined, and the results revealed that the autoclave had a vital role in driving the reaction to form FeF₃ nanomaterials. The charge capacity of the as-prepared FeF₃/C nanocomposites was 134.3, 103.2, and 71.0 mAh g^-1 at different current densities of 100, 500, and 1000 mA g^-1, which are superior compared to the bare FeF₃ materials and displayed stable cyclic performance with a charge capacity of 200 mAh g^-1.