Introduction

Fluorine is the most electronegative element in the periodic table. As a result, fluoride anion is very redox-stable, making it a suitable ion for charge transfer in a battery. When the electrochemical cell is either charged or discharged by the electric circuit of the battery, it is the transfer of fluoride ions that enables the reversible storage of electrons^[1]. Figure 1 is a visual representation of how fluoride-ion batteries (FIBs) work. In this example, BiF₃ and Mg were chosen as the electrodes. The oxidation of the anode (Mg --> MgF₂) will release two electrons. The cathode (metal fluoride) will be reduced to the corresponding metal by these two electrons. To balance the overall charge, F^- will be moved through the electrolyte. Since many metal fluoride salts that can be used as cathodes are either bivalent or trivalent, they could release several electrons per atom this way. As a result, high gravimetric density is achieved. High volumetric energy can also be achieved due to the high weights of the metal fluorides ^[1]. The electrochemical reactions taking place in FIBs are as follows:

Cathode reaction: MF_x + xe^- <--> M + xF^-

Anode reaction: M’ + yF^- <--> M’F_y + ye^-

Figure 1. Working principle of FIB, having BiF₃ as the cathode and Mg as the anode. (Figure: Azin Alesafar)

The principle of the fluoride-ion battery

Storage of the ions in electrodes can happen through two types of reaction mechanisms, conversion, and intercalation mechanisms. FIBs being suitable for reversible energy storage was first shown for cells with conversion-based electrodes. However, in a conversion reaction, and in this case during fluorination/de-fluorination the electrode material undergoes a redox reaction. As a result of this, a change in the crystal structure and hence a large volume change will happen in the electrode material^[2]. As Figure 2 displays, this volume change can lead to a reduction of physical connection between the electrode materials, the ion conductor, and the conductive particle and consequently result in a lower efficiency of the electrode material.

The second mechanism for storing the fluoride ions is the intercalation mechanism. In this type of reaction, the intercalation-based electrode material will be considered a host network, allowing guest ions to be stored in the unoccupied positions in the host lattice. The advantage of this mechanism is that the ions can reversibly be inserted and removed from the host material with lower volume changes during the redox reaction. As a result, compared to conversion-based electrode materials, intercalation-based materials often provide better electrochemical performance and better charging/discharging rates^[3]. The drawback of these materials on the other hand is their low specific capacities because intercalation-based materials often have higher weight than conversion-based materials^[4]. Specific capacity describes an electrode's performance. Specific capacity describes an electrode’s performance and defines the amount of electric charge (mAh) the material can deliver per gram of material. Figure 3 is a schematic of the conversion and intercalation mechanisms for lithium-ion batteries. 

Figure 2. Illustration of the changes in the contact surface area of the electrode material and electrolyte during the electrochemical charge/discharge. (Figure: Azin Alesafar, inspired by: ^[2])

Figure 3. Schematic of conversion (a) and intercalation (b) reactions for Lithium-ion batteries (License: CC BY 4.0). ^[5]

Components of the fluoride-ion battery

Taking the theoretical performance into consideration, there is a large number of possible cathode/anode pairs that perform suitable capacity, volumetric energy density and gravimetric energy density. However, the combinations need to be experimentally studied. Gschwind et al. ^[6] studied 63 possible cathode/anode combinations using room-temperature FIB.

Electrodes

Conversion-based cathode materials

BiF₃ and CuF₂ are the most used cathode materials due to their desirable ionic conductivity. However, further studies are needed to overcome the insufficient electronic conductivity of those. For example, Liu et al. ^[7]designed and constructed BiF₃/Bi₇F₁₁O₅ composites as an advanced cathode material for FIBs.

Intercalation-based cathode materials

Until now, three structures of Ruddlesden-Popper (K₂NiF₄-type structure), Schafarzikite (MSb₂O₄), and perovskite (AMO_3-y) have been studied as potential candidates for intercalation-based cathode materials for FIBs. Among the mentioned candidates, the Ruddlesden-Popper type compound appears to be the most suitable ^[2]. To elaborate on these compounds, we have to take account its structure which can be regarded as a perovskite-related structure, having a general formula of A_n+1M_nO_3n+1 or (AMO₃)_nAO where n is the number if connected layers. Figure 4 is a schematic illustration of the Ruddlesden-Popper structure when n=1, which is also known as K₂NiF₄-type structure. In a K₂NiF₄-type structure, MO₆ octahedra create AMO₃ (2D) perovskite layers and the AO rock salt units exist between the layers of the perovskite ^[2]. This will result in a tetragonal unit cell with the space group I4/mmm^[8]. The vacant anion positions in between the rock salt-type components are the proper places for the fluoride ions to be placed. K₂NiF₄-type compounds can accept a maximum of 2 fluoride ions per formula unit, resulting in theoretical capacities (normally between 130–160 mA h g^-1) comparable to various lithium-ion batteries ^[2]. 

Figure 4. Schematic illustration of the Ruddlesden–Popper
(A₂MO₄) structure (n=1) (left) and fully fluorinated A₂MO₄F₂ structure (right). (Figure: Azin Alesafar. inspired by ^[2]) 

Conversion-based anode materials

Significant studies related to the development of conversion-based electrodes have been conducted by Reddy and Fichtner^[9]. Composites based on Mg/MgF₂ ^[10], Ce/CeF₃ ^[9][10], and Ca/CaF₂^[11] have been intensively investigated as candidates for the anode materials of FIBs.

Intercalation-based anode materials

Not many studies have been done about intercalation-based anode materials for the FIBs. Hartman et al. ^[12]showed that layered lattice materials like Ca₂N and Y₂C and their transformation to fluorinated forms Ca₂NF and Y₂CF can work as anode materials for FIBs. Figure 5 shows the layered structure of Ca₂N.The material lattice parameters are a=b=3.62710 Å and c=18.97190 Å, it is trigonal and has the space group R-3m.

Figure 5. VESTA visualization of Ca₂N. Figure: Azin Alesafar.

Electrolytes

FIBs can be categorized based on their electrolytes. "All-solid FIB" or "high-temperature FIB (HFIB)" has a solid electrolyte and requires a high working temperature and the "room-temperature FIB (RTFIB)" that works in ambient conditions and has a liquid electrolyte ^[2].

Solid electrolytes

Most of the investigations on solid electrolytes for FIBs were conducted on single crystals some of which, show a high ionic conductivity. For instance, barium-doped lanthanum fluoride shows a fluoride ion conductivity in the range of 10^-4 S cm^-1. This order of ion conductivity shows that the mentioned material is a suitable solid electrolyte. However, since single crystals have complex growth processes, they are expensive, and integrating them into an all-solid battery configuration is quite difficult. Therefore, polycrystalline materials would be a more realistic substitute for solid electrolytes. The fluoride-conducting solid electrolytes can be categorized into two groups: Tysonite-type (LaF₃) and Fluorite-type (CaF₂). Different materials have been discussed to achieve improved solid-state electrolytes for FIBs. Some examples would be: rare-earth fluorides (tysonite-type) doped with alkaline-earth or alkaline-earth fluorides (fluorite-type) doped with rare-earth fluorides ^[2].

Liquid electrolytes

Although fluoride-ion batteries have been mostly investigated based on their solid electrolytes and the operating temperature of 150 °C, more studies have been conducted focusing on FIBs having liquid electrolytes. There are two main methods to prepare fluoride ion-conducting liquid electrolytes. The first is by dissolving fluoride salts in organic solvents and the second method is mixing organic fluorides into ionic liquids. So far, only one patent has stated the preparation of FIB cells using the ionic liquid-based electrolyte. The case was mixing tetramethylammonium fluoride (TMAF) in 1-methyl-1-propylpiperidinium bis(tri-fluoromethanesulfonyl)imide (MPPTFSI)^[13]. A liquid fluoride conducting electrolyte having high ionic conductivity and wide electrochemical stability was designed by Davis et al^[14]. The electrolyte (concentration>2.2 M) was synthesized by dissolving dry N,N,N,trimethyl-N-neopentylammonium fluoride (Np1F) in the organic solvent Bis(2,2,2-trifluoroethyl) ether (BTFE). They showed that the 0.75M Np1F/BTFE electrolyte displays high ionic conductivity of 7.95 mS cm^-1.

Table 1 summarizes some of the electrochemical properties of the anode/cathode combinations with respective electrolytes.

Table 1. Electrocemical properties of some of the studied materials combinations for FIBs

Outlook

Theoretical studies show the potential of different combinations of metal fluoride salts with pure metals as anodes, proving the possibility of FIBs. Further investigations should take place for complex fluoride salts such as ammonium hexafluorophosphate. For FIBs to be brought into the market, a safety assessment should also be done. So far, the studies show that FIBs do not represent any major issues in terms of safety issues. Among the various studied combinations for the FIB components, battery combinations based on alkaline (earth) metals as the anode were the most promising, and combinations including Mg or Al also showed good performance. In conclusion, fluoride ion batteries are a promising technology that has the potential to revolutionize the energy storage industry. With their high energy density and long cycle life, they offer advantages over traditional lithium-ion batteries and are suitable for a wide range of applications. Finding the best materials to use in fluoride-ion batteries is still a challenge as it requires numerous experiments and analyses. Jack Sundberg and colleagues in Scott Warren's laboratory at the University of North Carolina at Chapel Hill, US^[17]using supercomputers, developed a machine learning method that is able to accurately and quickly predict how fluoride ions move in different crystals containing fluoride. They could rank structures based on their fluoride-transporting ability. In the first step, they cut down a data base of 140000 known materials to 10000 fluoride-containing candidates. Then they randomly selected 300 of the 10000 candidates and then ran precise calculations for each material's fluoride-transporting ability. The calculations took a week per material. These calculations were later used to train the system to develop faster calculations. Final algorithms could be run in one hour for each material. Their studies showed ZnTiF₆ as a promising electrolyte for a fluoride-ion battery. Other studies should be conducted to fully investigate the capabilities of fluoride-ion batteries and to discover opportunities that these batteries could be used instead of lithium-ion batteries.

References