The case for fluoride-ion batteries

Recently reversible batteries based on fluoride ions as a charge carriers have attracted some attentions as an alternative electrochemical energy storage system to conventional lithium ion batteries (LIBs). Fluoride is the most stable anion with a high mobility and therefore, fluoride ion batteries (FIBs) can theoretically provide a wide electrochemical potential window. Moreover, FIBs are capable of being built in an all solid-state modification. Previously, electrochemical fluoride ion cells based on conversion-based electrode materials have been built. However, the state of the art of the FIBs suffer from poor cycling performance in lack of well-developed cell components including the electrode materials. In the current study, intercalation-based cathode materials have been investigated as an alternative approach to make electrode materials for FIBs. In this respect, various compounds with mainly Ruddlesden-Popper-type structure including LaSrMO4 (M = Mn, Co, Fe) and La2MO4+d (M = Co, Ni) as well as Schafarzikite-type compounds of Fe0.5M0.5Sb2O4 (M = Mg, Co) have been subjected to electrochemical measurements including galvanostatic cycling, cyclic voltammetry and electrochemical impedance spectroscopy and the structural changes upon electrochemical fluorination/de-fluorination were analyzed by X-ray Diffraction (XRD). LaSrMnO4 has been fluorinated/de-fluorinated via electrochemical method confirming successful intercalation/de-intercalation of the fluoride ions, but showed problems for long-term operation. In contrast, La2NiO4+d showed to be the most promising intercalation-based cathode material (for FIB) in terms of cycling stability (>220 cycles and 60 cycles for cutoff capacities of 30 and 50 mAh/g, respectivly) with a nearly 100% Coulombic efficiency (average Coulombic efficiency of 97.68% and 95.44% for cutoff capacities of 30 and 50 mAh/g, respectively). This is the highest cycle life that has been reported so far for a FIB. One of the major challenges of the proposed FIB systems was found in avoiding oxidation of the conductive carbon which has been mixed with the electrodes to improve the electronic conductivity. This decomposition of the carbon matrix results in a remarkable increase in the impedance of the cell and can significantly impair the cycle life and discharge capacity. However, the critical charging conditions which could be determined by cyclic voltammetry and electrochemical impedance spectroscopy have a major impact on preserving the conductivity of the cell. In addition, the effect of volume change in the conversion-based anode materials has been studied showing that the overpotentials arising from the volume change can significantly influence the cycling behavior of the battery system (due to absence of well-developed intercalation-based anode materials for FIBs, conversion-based counter electrodes have been used as anode materials).

Fluoride ion batteries (FIBs) have recently gained much attention as a next generation battery system because of their potential to surpass lithium-ion batteries (LIBs) in many aspects such as high energy density and low cost. FIBs with solid or liquid electrolyte have been studied intensively, amongst liquid-based FIBs has an advantage of good interfacial contact between the electrolyte and electrode, and low-temperature operation over all-solid-state FIBs. However, stability has been a major concern for liquid electrolytes. The instability caused by the strong basicity of F− ion attacking most of the commonly used organic solvents leading to side reactions.1 So far bis(2,2,2-trifluoroethyl) ether (BTFE), which has good base resistance, has been proposed as an effective organic solvent for electrolyte.2 However, the boiling point of BTFE is relatively low (64 °C). While looking beyond organic solvents, ionic liquids (ILs) can be a choice as an electrolyte for FIBs owing to their good properties such as nonvolatility and wide electrochemical window.3 It is commonly known that quaternary ammonium cations undergo Hofmann elimination, during which β-hydrogen is withdrawn by strong bases such as OH− and F− ions.4,5 Therefore, to use ILs as an electrolyte for FIB, it is necessary to improve their stability against F− ion. One way to weaken the basicity of F- ion is to solvate them by a complexing agent. Hagiwara et al. reported that imidazolium-based ILs containing ethylene glycol are stable despite the presence of F− ion.6 Bond formation between F− ion and hydrogen in the hydroxy groups of ethylene glycol and imidazolium-based cation prevents the F− ion from attacking the β-hydrogen of the cation's alkyl chain. However, there have been no reports of FIBs using ILs containing solvated F− ion as electrolytes. In this study, we investigated the thermal stability of choline bis(trifluoromethanesulfonyl)amide (N111(2OH) TFSA), an IL with hydrogen bond donor functional group, against F− ions. Furthermore, we performed charge-discharge tests on liquid-based FIBs using N111(2OH) TFSA containing fluoride salt as an electrolyte.N111(2OH) TFSA (Fig. 1 (a)) was synthesized by ion exchange of N111(2OH) Cl with K TFSA. The melting point of the synthesized N111(2OH) TFSA was estimated to be 38 °C from DSC measurement and it was solid at room temperature. 0.4 mol kg−1 tetramethylammonium fluoride (TMAF) in N111(2OH) TFSA was found to be stable up to about 150 °C (Fig. 1(c)). TGA measurements of N,N,N-trimethyl-N-propylammonium (N1113 TFSA) (Fig. 1(b)) without hydroxy groups were also performed under the same conditions for comparison. A solution of 0.4 mol kg−1 TMAF in N1113 TFSA showed a gradual weight loss right after the measurement was started, with a significant weight loss at about 120 °C (Fig. 1(c)). These results suggest that N111(2OH) TFSA with the hydroxy group exhibits higher thermal stability toward F− ion. This was attributed to the hydrogen bonding of the hydroxy group to F− ion, which weakened the basicity of F− ion. Finally, charge-discharge tests were performed in a three-electrode cell using BiF3 working electrode, Pb counter electrode, and Ag/Ag+ reference electrode. Figure 1 (d) shows charge-discharge curves of BiF3 electrode with 0.4 mol kg−1 TMAF in N111(2OH) TFSA as an electrolyte at 60 °C. The first discharge capacity of the BiF3 electrode was 300 mAh g−1, which represents 99 % of the theoretical capacity of BiF3 (302 mAh g−1) with a columbic efficiency of 85%. This result suggests that the BiF3 was reversibly defluorinated/fluorinated with 0.4 mol kg−1 TMAF in N111(2OH) TFSA.AcknowledgmentThis presentation is based on results obtained from a project, JPNP21006, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).References(1) V. K. Davis, et al., Mater. Chem. Front., 2019, 3, 2721-2727.(2) V. K. Davis, et al., Science, 2018, 362, 1144–1148.(3) K. Okazaki, et al., ACS Energy Lett., 2017, 2, 1460–1464(4) S. Raiguel, et al., Green Chem., 2020, 22, 5225-5252.(5) H. Sun, and S. G. Dimagno, J. Am. Chem. Soc., 2005, 127, 2050-2051.(6) Z. Chen, et al., J. Phys. Chem. Lett., 2018, 9, 6662−6667 Figure 1

Fluoride-ion batteries: State-of-the-art and future perspectives

Due to the limitations of lithium‐ion batteries (LIBs), there is an urgent need to explore alternative energy storage technologies. However, the high‐energy density of fluoride‐ion batteries (FIBs) has attracted widespread attention as a potential successor to LIBs. FIBs are emerging as a low‐cost, safe, and versatile energy storage solution, with a broad operating temperature range. With continuous efforts from researchers, significant progress has been made in the field of FIBs. Nevertheless, compared to traditional batteries, research on FIBs remains limited, and many challenges and unexplored avenues persist. This article elucidates the principles of FIBs, summarizes the materials for both cathodes and anodes, discusses electrolytes, and addresses existing issues. It also outlines future directions and potential applications of FIBs. As it is continued to innovate and explore, FIBs hold promise for revolutionizing energy storage technology, offering enhanced performance, safety, and sustainability.

Fluoride ion batteries (FIBs) are among interesting electrochemical energy storage systems that are being considered as alternatives to lithium-ion batteries (LIBs). FIB offers high specific energy and energy density, thermal stability, and safety. Despite the advantages posed by the FIBs, several challenges need to be addressed to realize its full potential. We have been working on various aspects related to FIB with the aim of developing sustainable fluoride ion batteries. So far rechargeable FIBs have been demonstrated only at an elevated temperature like 150 °C and above. Here, for the first time, we demonstrate room-temperature (RT) rechargeable fluoride-ion batteries using BaSnF4 as fluoride transporting solid electrolyte. The high ionic conductivity of tetragonal BaSnF4 (3.5 × 10–4 S cm–1) enables the building of RT FIB. We built fluoride ion batteries using Sn and Zn as anodes and BiF3 as a cathode. We have investigated the electrochemical properties of two different electrochemical cells, Sn/BaSn...

Rechargeable batteries with high energy density have been required for recent applications such as battery electric vehicles. Fluoride ion batteries (FIBs) can achieve high energy density by using monovalent fluoride ions as charge carriers and multi-electron reactions of electrode active materials, and have attracted much attention as a candidate to surpass the performance of the current lithium-ion battery (LIB). However, FIB using the metal/metal fluoride conversion reaction, which has a high potential for energy density, has not been fabricated with good reversibility as the current LIB. On the other hand, the FIB using a crystal capable of intercalation (insertion-extraction) reaction of fluoride ions as the active material has been developed [1]. Recently, an all-solid-state fluoride ion battery using an oxyfluorosulfide (Sr2F2Fe2OS2; SFFOS) as the intercalation-type positive active material has been reported, and the battery has operated with high capacity and good reversibility at 413 K [2]. In this study, SFFOS was evaluated as an active material for liquid-based FIB. In particular, the thermal effect on the intercalation reaction of SFFOS in ionic liquid-based electrolyte solution was measured.The SFFOS was synthesized from four kinds of powder, SrF2, SrO, Fe and S, with molar ratio of 1:1:2:2 referring to the previous papers [2, 3]. These powders were mechanically ground under argon and pressed at 6 MPa to form a pellet. The resulting pellet was vacuum sealed in a quartz tube and sintered at 1073 K for 36 hours to obtain the SFFOS. PbSnF4 as the counter electrode material was prepared from PbF2 and SnF2 by mechanical milling at 600 rpm for 6 h using a planetary ball mill, and then calcined at 673 K for 1 h under an argon atmosphere. The electrolyte solution was prepared by dissolving anhydrous tetramethylammonium fluoride (TMAF) in an ionic liquid, N,N,N-trimethyl-N-propylammonium-bis(trifluoromethanesulfonyl)amide (TMPA-TFSA), at various concentrations such as 0.075 mol/dm3 (the molar ratio of TMAF:TMPA-TFSA=1:50). The working and counter electrodes were prepared by mixing of SFFOS or PbSnF4 with acetylene black and PVdF, respectively. The galvanostatic intercalation reaction of fluoride ions into SFFOS was measured using a three-electrode cell with a Pb|PbF2 reference electrode at 298, 373, and 423 K.The galvanostatic intercalation reaction (corresponding to a charging reaction) of the SFFOS/AB/PVdF composite electrode showed the charging plateau at about 0.5 V vs. Pb|PbF2. The following deintercalation (discharging) reaction, the discharge plateau was observed around 0 V. The intercalation/deintercalation plateaus were similar to those of the previously reported all-solid-state FIB, well [2].

Fluorine is highly electronegative (+2.84 V vs. SHE) element and possess low weight. Fluoride is monovalent, and it has high charge density. Consequently, it enables high ionic conductivity similar to Li+ in certain solids. However, fluorine is a corrosive and toxic gas, which forbids it as one of the electrodes. Nevertheless, fluoride ions can be transported between a metal and a metal fluoride through a fluoride transporting electrolyte, and fluoride-ion batteries (FIB) can be realized. FIB offers high specific energy, energy density, thermal stability, and safety compared to state of the art lithium-ion batteries (LIBs). Primary batteries based on fluoride transport were demonstrated earlier [1]. Recently we have demonstrated first rechargeable FIB that was operable at 150 °C [2]. Several challenges need to be addressed to realize its full potential of FIBs. We have been working on various aspects related to FIB with the aim of developing sustainable fluoride ion batteries [3-5]. Recently, for the first time, we have demonstrated room-temperature (RT) rechargeable fluoride-ion batteries using BaSnF4 as fluoride transporting solid electrolyte [6]. BaSnF4 shows high fluoride conductivity of 3.5x10–4 S cm–1, but limited electrochemical stability window. To be compatible with BaSnF4, we have utilized low electropositive Sn, Zn as anode materials to demonstrate RT FIB. However, to enable cells with high operating potentials, the electrolyte should be compatible with highly electropositive metals (e.g., Mg, Ce). Toward this, we developed a new interlayer electrolyte that is compatible with highly electropositive metals like Ce and Mg. Further, utilizing the interlayer electrolyte, we demonstrate the functioning of first high potential RT FIB.

Chloride ion battery: A new emerged electrochemical system for next-generation energy storage

Further enhancements regarding the energy density and specific power of lithium ion batteries are absolutely necessary in order to satisfy the increasing requirements for automotive applications e.g. extended driving ranges. One way to realize such improvements, depicts the replacement of commonly used carbon-based anode materials with high capacity anodes.1 In this regard silicon (Si) containing composites are considered promising candidates for the replacement of carbonaceous anode materials due to the significantly higher specific capacity of Si. However, the use of Si is still hindered by several challenges that have to be overcome for a successful application. One major issue of Si-based anode materials is the strong capacity decay due to the huge volume changes (~ 300%) of Si during the lithiation/de-lithiation process, leading to an recurring solid electrolyte interphase (SEI) reformation, which results in the ongoing consumption of active lithium from the cathode.2 One concept to alleviate the detrimental effects previously mentioned and to boost the performance of such anode materials, comprises the combination of Si with a matrix material. The general idea behind this approach is to combine Si with a second phase that enhances the mechanical stability and can buffer the volumetric changes of Si and thus, enables the formation of a stable SEI.3,4 In this study, we present a silicon iron (Fe) composite that contains two phases, a crystalline Si phase and a second, intermetallic FexSiy-phase. The applied synthesis route yields materials that combine a porous structure with the aforementioned matrix approach. This composite design is believed to have a beneficial effect on the capacity retention during cycling since it may buffers the volumetric changes of the Si and simultaneously provides extra space for the volume changes. The applied synthesis route contains a ball-milling and washing step, and subsequently the addition of a thin carbon coating. The composites, synthesized this way, are investigated via scanning electron microscopy, energy dispersive x-ray spectroscopy, x-ray diffraction and thermogravimetric analysis in order to characterize their structure, morphology and composition. Moreover, electrochemical studies on the long-term cycling and rate performance with regard to the application as anodes in lithium ion batteries are conducted. Thereby, this work focuses on the influence of a high temperature treatment, as well as the influence of the Fe to Si ratio on the electrochemical performance. Furthermore, the role of the stabilizing FexSiy matrix phase in the composite is investigated regarding the question whether this phase is inactive towards lithiation or if it contributes to the lithiation/de-lithiation capacity of the composite. References 1 Placke, T.; Kloepsch, R.; Dühnen, S.; Winter, M. Lithium ion, lithium metal, and alternative rechargeable battery technologies: The odyssey for high energy density. Journal of Solid State Electrochemistry 2017, 21, 1939-1964. 2 Wu, H.; Cui, Y. Designing nanostructured Si anodes for high energy lithium ion batteries. Nano Today 2012, 7, 414–429. 3 Besenhard, J. O.; Yang, J.; Winter, M. Will advanced lithium-alloy anodes have a chance in lithium-ion batteries? Journal of Power Sources 1997, 68, 87–90. 4 Park, C.-M.; Kim, J.-H.; Kim, H.; Sohn, H.-J. Li-alloy based anode materials for Li secondary batteries. Chemical Society reviews 2010, 39, 3115–3141.

A fluoride-ion battery (FIB) is a novel type of energy storage system that has a higher volumetric energy density and low cost. However, the high working temperature (>150 °C) and unsatisfactory cycling performance of cathode materials are not favorable for their practical application. Herein, fluoride ion-intercalated CoFe layered double hydroxide (LDH) (CoFe-F LDH) was prepared by a facile co-precipitation approach combined with ion-exchange. The CoFe-F LDH shows a reversible capacity of ∼50 mAh g-1 after 100 cycles at room temperature. Although there is still a big gap between FIBs and lithium-ion batteries, the CoFe-F LDH is superior to most cathode materials for FIBs. Another important advantage of CoFe-F LDH FIBs is that they can work at room temperature, which has been rarely achieved in previous reports. The superior performance stems from the unique topochemical transformation property and small volume change (∼0.82%) of LDH in electrochemical cycles. Such a tiny volume change makes LDH a zero-strain cathode material for FIBs. The 2D diffusion pathways and weak interaction between fluoride ions and host layers facilitate the de/intercalation of fluoride ions, accompanied by the chemical state changes of Co2+/Co3+ and Fe2+/Fe3+ couples. First-principles calculations also reveal a low F- diffusion barrier during the cyclic process. These findings expand the application field of LDH materials and propose a novel avenue for the designs of cathode materials toward room-temperature FIBs.

Rational design on materials for developing next generation lithium-ion secondary battery

Lithium ion batteries (LIBs) are the dominating energy storage devices in the field of portable consumer electronics. They are also considered to be the most promising technology for the application in electric vehicles due to their high energy density. Nonetheless, their power and energy density need to be further improved to meet the challenging requirements for automotive applications, such as extended driving ranges and fast charging capabilities. [1,2] The partial substitution of the commonly used anode material graphite with silicon (Si) depicts one possible approach to increase the energy density significantly, since Si offers a higher specific capacity compared to graphite. However, an insufficient capacity retention of Si-based anodes during cycling represents a major challenge, regarding the commercial application. The poor cycling performance is caused by enormous volume changes, accompanying the lithiation/de-lithiation process of Si, resulting in the deterioration of the electrode due to pulverization of Si particles and contact loss between the active material and the current collector. Additionally, the consumption of active lithium and electrolyte during the formation of a new passivating solid electrolyte interphase (SEI) in every cycle contributes to a strong capacity decay. [3] The performance of Si-based anodes can be improved by embedding Si in different matrices, e.g. graphite or amorphous carbon. Furthermore, the alloying of Si with an inactive metal, such as iron (Fe) can significantly increase the performance by forming electrochemical inactive metal silicides that can stabilize the whole structure and suppress volume changes. [4, 5] In this contribution, we present the synthesis of Fe/Si-alloys via a simple high energy ball milling process, using elemental Fe and Si as the precursor materials. The influence of different Fe:Si ratios, the addition of carbon, as well as the influence of different heat-treatment conditions on the structure and electrochemical performance are investigated. Therefore, the synthesized active/inactive Si/FexSiy-composites are analyzed by X-ray diffraction and scanning electron microscopy equipped with energy dispersive X-ray spectroscopy in order to identify the formed intermetallic phases, their morphology and elemental distribution within the material. Nitrogen adsorption experiments are carried out to determine the specific surface area of Si/FexSiy-composites. To evaluate the suitability of the composites as high-energy anode in LIBs, their electrochemical performance is characterized regarding their long-term cycling stability and rate capability. References 1 Placke, T.; Kloepsch, R.; Dühnen, S.; Winter, M. Lithium ion, lithium metal, and alternative rechargeable battery technologies: The odyssey for high energy density. Journal of Solid State Electrochemistry 2017, 21, 1939-1964. 2 Andre, D.; Hain, H.; Lamp, P.; Maglia, F.; Stiaszny, B. Future high-energy density anode materials from an automotive application perspective, Journal of Materials Chemistry A 2017, 5, 17174-17198. 3 Wu, H.; Cui, Y. Designing nanostructured Si anodes for high energy lithium ion batteries. Nano Today 2012, 7, 414–429. 4 Besenhard, J. O.; Yang, J.; Winter, M. Will advanced lithium-alloy anodes have a chance in lithium-ion batteries? Journal of Power Sources 1997, 68, 87–90. 5 Obrovac M. N., Chevrier V. L. Alloy Negative Electrodes for Li-Ion Batteries, Chemical Reviews 2014, 114, 11444-11502

Last developments in polymers for wearable energy storage devices

A flexible tysonite-type La0.95Ba0.05F2.95@PEO-based composite electrolyte for the application of advanced fluoride ion battery

Solid-state synthesis and ion transport characteristics of the β-KSbF4 for all-solid-state fluoride-ion batteries