Research report on new type of fluoride ion battery

1. Comparison between lithium ion batteries VS fluoride ion battery
2. Selection of electrode materials
   1. Conversion electrodes in fluoride ion battery
   2. Embedded electrodes in fluoride ion battery
3. Fluoride ion electrolyte
4. Conversion reactions using lithium-ion batteries as an example
5. Key research fronts
6. Summary

In recent years, fluoride ion batteries have emerged as a new area of research with the potential to meet future electrochemical energy storage needs. Compared with lithium-ion batteries, fluoride ion battery utilizes fluoride anions (F-) instead of lithium cations (Li+) as the main charge carriers.

Fluorine is the most electronegative element, and the fluoride anion exhibits high oxidative stability, allowing the use of higher voltage redox pairs. Similar to lithium, fluorine is the smallest and lightest element in its class, enabling faster ion transport and higher energy density than comparable anionic chemistries such as Cl−.

The annual production of fluorine in the world exceeds 3.5 million tons, which is nearly two orders of magnitude higher than that of lithium. Thanks to the abundant reserves of fluorine and the perfect supply chain, fluoride ion battery is expected to become a low-cost energy storage system to replace LIB technology. In addition, fluoride ion battery has higher mass and volumetric energy densities due to multi-electron redox reactions. And fluoride ion battery has better safety without the problems of dendrite growth and exothermic oxygen decomposition.

1. Comparison between lithium ion batteries VS fluoride ion battery

Research on fluoride ion battery is still in its infancy, and while they may avoid some of the problems of LIBs, fluoride ion battery also poses a new set of challenges. These are mainly related to the stability and conductivity of the F-ion electrolyte, as well as the available capacity and reversibility of the electrode redox reactions.

Recently, some experts have proposed the most promising and feasible conversion-type cathode and anode materials from the perspectives of capacity and electrode potential. This report will compare the energy density and cost of conversion and intercalation fluoride ion battery with Li-ion and high-energy density Li-metal batteries.

The potential commercial value of conversion liquid fluoride ion battery cells is highlighted, with energy densities as high as 588 Whkg-1 (1,393 WhL-1) and costs as low as $20 kWh-1 at the stack level. This perspective highlights the main obstacles hindering the development of fluoride ion battery and draws relevant lessons from research on Li-ion batteries.

To practically advance fluoride ion battery as a viable technology, further research must focus on the development of safe fluoride electrolytes and SEI formation. Do you know about the differences between fluoride battery electrolyte and lithium battery electrolyte? In the following we will introduce the electrolyte of fluorine and you can read our lithium ion battery electrolyte to know clearly about the differences between them.

2. Selection of electrode materials

Using fluoride ions as carriers, fluoride ion battery is a competitive battery technology, and its performance mainly depends on the properties of the electrode materials. A battery releases energy when electrons are transferred from a material with a high Fermi level (anode) to a material with a low fermi level (anode).

In fluoride ion battery, electrical neutrality is maintained by simultaneously extracting fluoride ions from the positive electrode material and intercalating fluoride ions in the negative electrode material. When designing a viable fluoride ion battery, electrodes must be chosen based on energy density, reversibility of the (de)fluorination reaction, and feasibility of production.

In short, to obtain high energy density, positive and negative electrode materials with large redox potential difference and low molecular weight should be selected; there are additional constraints on the reversibility and feasibility of electrode materials. Furthermore, materials with the smallest volume change during fluorination and defluorination should be preferentially selected to reduce internal stress and alleviate pulverization, contact loss between active material and electrode, and electrode/electrolyte interface degradation.

The active material should have very low solubility in the electrolyte to prevent self-discharge and continuous capacity loss. Excellent electrical conductivity can reduce the need for small particle size and large proportions of conductive additives for active materials. Ultimately, materials that meet these conditions should also have reasonable costs and a reliable supply chain to produce a competitive technology.

1. Conversion electrodes in fluoride ion battery

The concept of fluoride ion battery was first proposed using conversion electrodes. In fluoride ion battery cells, the conversion reaction involves the electrochemical conversion between any metal and its corresponding metal fluoride M + xF− ↔ MFx + xe−. A full battery in a discharged state uses metal as the positive electrode and metal fluoride as the negative electrode.

The difference between the two electrodes is the relative redox potential of their constituent metals. While a large number of compounds are potentially useful as fluoride conversion electrodes, only those with extreme electrode potentials are technologically relevant. In addition to the above criteria, the crystal structure of the relevant fluoride phase should also be considered, as this will affect the ionic conductivity and ultimately the electrochemical performance of the active material.

Two of these crystal structures are believed to be superionic conductors at high temperatures, namely the cubic fluorite structure (Fm3¯m) common to several alkaline earth metal fluorides (CaF2, BaF2 and SrF2) and many rare earth fluorides (LaF3, CeF3 and NdF3) has a triangular tysonite structure (P3¯c1). Since they are usually composed of low work function metals, they can be used as conversion-type anode materials.

3D transition metal fluorides are generally considered to be the most energy-dense and most economical conversion cathode materials. The CuF2 cathode combines the advantages of high redox potential, low molecular weight, and low cost. In fluoride ion battery, the reversible (de)fluorination of Cu/CuF2 is achieved and verified in many different forms. NiF2, FeF3 and SnF4 are also suitable cathode materials for high energy density applications, their cathode potential is significantly lower than that of CuF2, but they have higher mass capacity.

Conversion-type anode materials consist of positively charged (low work function) metals of which there are three main groups: alkali metals, alkaline earth metals, and rare earth metals. Alkali metal fluorides are not ideal candidates because they are generally more soluble in liquid electrolytes.

Furthermore, they crystallize only in the rock salt (Fm3m) structure, which exhibits poor F− ionic conductivity and is limited to single electron transfer per metal atom. Divalent and trivalent metal fluorides exhibit lower solubility because of their higher lattice energies due to greater charge density and smaller cation size. Unlike switching cathodes, many switching anodes do exhibit F-ion conducting crystal structures.

In many respects, CaF2 is the most ideal converted F− ion anode. CaF2 has a reduction potential of ~0 V relative to Li+/Li and a capacity of 686.5 mAh g-1, which is the material with the highest energy density among fluoride ion battery anodes. Although MgF2 has comparable energy densities, CaF2 has a favorable fluorite crystal structure and almost no volume change upon (de)fluorination. YF3 and SrF2 have energy densities second only to CaF2 and MgF2. While comparable in terms of energy density and cost, they present different challenges. Similar to CaF2, SrF2 has a fluorite crystal structure and small volume change, but significantly lower capacity.

The low redox potential of these materials lies on the edge of the electrochemical stability window of many liquid electrolytes, making them difficult to exploit. The anode potential of YF3 is relatively suitable, but it has no F-conducting crystal structure and huge volume change.
Many cathode materials have been found, including MgF2, CaF2, LaF3, and CeF3.

None of the current battery performance is optimal, and it appears that electrode preparation and electrolytes have a greater impact than the intrinsic properties of the electrode material. To limit the deleterious effects of extrinsic factors, close attention must be paid to active material particle size, carbon composite structure, binder and separator chemistry, and electrochemical stability of electrolytes.

2. Embedded electrodes in fluoride ion battery

Compared with conversion electrodes, these fluoride intercalation electrodes have the following advantages: higher cathodic potential and lower (de)fluorination volume change, deintercalation reactions generally also exhibit excellent rate capability and longer cycling lifetimes because they rely only on the diffusion of one species and do not require the formation and migration of metal/metal fluoride interfaces.

Electrochemical fluoride intercalation has only been experimentally validated in all-solid-state (ASS) batteries, which typically require elevated temperatures (140°C to 200°C) to enhance the F-ion conductivity of solid electrolytes. The data show that the explored fluoride intercalation type has a layered structure or a tunnel structure, as well as the fluorination of the perovskite BaFeO2.5. Generally, fluoride intercalation electrodes store charge through redox of 3D transition metals.
However, to accommodate fluoride intercalation, multiple high-molecular-weight cations and oxyanions are required per redox center of the host lattice. This inactive structure severely limits the mass capacity of such materials. Unlike intercalation cathodes, very few intercalation anode materials have been identified. The only experimentally investigated compound is Sr2TiO3F2, and the (de)fluorination is limited to chemical methods.

Although fluoride-intercalated electrodes can offer higher rate performance and cycle life than conversion-type compounds, they may face greater safety concerns. Ultimately, the success of fluoride intercalation compounds does not depend on superiority over fluoride conversion chemistry, but in traditional lithium-ion battery technology.

3. Fluoride ion electrolyte

Solid electrolytes are considered to be a major breakthrough in the development of LIBs. In LIBs technology, solid electrolytes have the following advantages: (1) inhibiting the growth of lithium dendrites, allowing the use of lithium metal anodes with high energy density; (2) non-flammable electrolytes, improving battery safety.

In fluoride ion battery, however, the dendrite problem does not exist. Although nonflammable solid electrolytes still have some safety advantages, the possibility of spontaneous exothermic reactions with electrolytes in fluoride ion battery is significantly reduced if oxygen-free fluoride cathodes are used.

Solid electrolytes for fluoride ion battery is actually problematic in many ways. Performance-wise, they typically require high temperatures (>~140°C) to operate, and even at such high temperatures, their ionic conductivity is relatively low. From a practical point of view, they are not suitable for conventional battery fabrication techniques, and it is difficult to achieve good interfacial contact between the solid electrolyte and electrode material. Furthermore, this interfacial contact typically fails if the active material undergoes a significant volume change, making solid electrolytes substantially incompatible with most conversion-type materials.

Perhaps most importantly, the higher density of solid-state electrolytes severely limits the energy density of solid-state batteries compared to liquid batteries. Wide electrochemical stability windows are easier to achieve using solid F-ion electrolytes, and the chemical reactivity is low, making them safer, easier to produce, and generally more stable. Further research should ultimately focus on improving these properties of liquid electrolytes. In “anode-free” batteries, solid electrolytes can be comparable to liquid electrolytes in terms of energy density.

A special challenge in developing high-voltage liquid fluoride electrolytes stems from the highly nucleophilic nature of F− ions. Due to electrostatic repulsion between lone pairs of electrons within its small ionic radius, F− ions are more prone to losing electrons than other halide ions. This Lewis alkalinity is especially severe in most organic battery solvents (anhydrous and aprotic), where the F-ion is unsolvated or “bare”. There are two main strategies for developing stable liquid electrolytes: (1) formulate electrolytes that are not attacked by nucleophilic F-; (2) solvate F-ions to reduce their alkalinity.

Battery safety is probably the biggest concern for liquid fluoride electrolytes. In addition to their potential flammability, liquid fluoride electrolytes are often highly toxic and corrosive due to the chemical reactivity of F-ions. It is unclear whether these properties can be addressed by engineering electrolyte chemistry and may require additional engineering safety measures to implement.

4. Conversion reactions using lithium-ion batteries as an example

While fluoride ion battery represent a new chemistry distinct from LIBs, it is still possible to draw on LIBs that have been studied for decades, especially translational electrodes. For conversion-type transition metal fluoride cathodes in LIBs, there are three main failure mechanisms: active material (transition metal) dissolution, electrolyte decomposition, and particle coarsening.

The first two failure mechanisms occur at the interface of the bare metal surface in contact with the electrolyte, a topographical feature also found in fluoride ion battery converted cathodes. Transition metal dissolution oxidizes at these metal/electrolyte interfaces and competes with (re)fluorination of metal particles. This dissolution will be mitigated if the fluoride surface formed upon oxidation is an effective passivation layer.

Significant volume changes associated with the transformation reaction may lead to disruption and remodeling of the SEI. This will depend on how the morphological evolution of the active material particles accommodates volume changes within the material. Furthermore, this SEI degradation can be mitigated if the SEI has some elasticity, such as a polymer SEI layer.

Another failure mechanism stems from the fact that metal surfaces can highly catalyze the decomposition of organic electrolytes. In extreme cases, SEI growth increases the charge transfer resistance, isolating the electrode from the electrolyte. Forming a strong SEI, or coating the surface of the active material with a carbon layer, can effectively alleviate the above problems.

Particle coarsening increases the distance for ion and electron transport, which is very disadvantageous for conversion electrodes with lower conductivity. In LIBs, coarsening occurs when lithium diffuses into the particle interface. Whether fluoride diffusion would cause the same effect is uncertain. Strategies to prevent coarsening include carbon coating, anchoring through direct chemical growth of active materials on carbon, and electrostatic separation using ionic electrolytes.

As with the conversion fluoride in LIB, the (de)fluorination reaction must depend on the formation and migration of the metal/metal fluoride interface. Whether these interfaces are coherent, semi-coherent, or incoherent will affect reaction rates, reaction overpotentials, and the optimal particle size for each active material.

5. Key research fronts

Fluoride ion battery is expected to be a low-cost, high-energy-density energy storage technology. However, their successful application requires substantial improvements to current technologies, which are mainly divided into the following three aspects:

•  Mechanistic exploration. Uncovering the charge/discharge mechanism of transformed fluoride electrodes is crucial to understanding their functions and limitations.
• Electrolyte design and characterization. Designing and developing suitable liquid fluoride electrolytes will be the key to fluoride ion battery as a competitive energy storage technology.

6. Summary

Fluoride ion battery is expected to develop into an energy storage technology with higher energy density. From a techno-economic point of view, many conversion fluoride ion battery chemistries can easily exceed the energy density of NMC811 lithium batteries at a fraction of the cost. Cu-CaF2 batteries are the most attractive in terms of energy density and cost.

However, the higher voltage of this redox pair also poses greater challenges for electrolyte development. The success of fluoride ion battery ultimately depends on the development of chemically and electrochemically stable liquid fluoride electrolytes that can form good SEIs. The projected energy density of current embedded fluoride ion battery is too low to be economically viable.

Unless high-capacity electrodes are found, embedded fluoride ion battery need to offer major advantages over traditional LIBs (eg, cost and life cycle) to be successful.  Especially the life cycle, the lithium ion battery has long life cycle, and do you know the reason why it has long life cycle? You can have a look at our article of lithium ion battery life cycle to get further information. Ultimately, the realization of high-energy-density fluoride ion battery will depend on future research results. These efforts should mainly focus on developing safe liquid electrolytes, exploring switching mechanisms, and constructing electrodes and batteries.

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