Spectacle-like SiX (X = P, As): A promising candidate for dual application in fluoride and magnesium ion battery from first-principles calculations

As an alternative to lithium‐based batteries, fluoride‐ion batteries (FIBs) offer a promising pathway owing to their high theoretical energy density. However, their progress has been hindered by the lack of room‐temperature electrolytes, either solid or liquid, that exhibit sufficiently wide electrochemical stability windows and that allow F‐ion shuttling between electrodes with low overpotential. In this work, we identify 1‐butyl‐3‐methylimidazolium fluoride (BMimF) dissolved in acetonitrile (MeCN) as a particularly promising electrolyte candidate. This system exhibits a high room‐temperature ionic conductivity of 15.3 mS cm −1 and an electrochemical stability window ranging from −0.37 to +1.59 V vs. Pb/PbF 2 . 19 F NMR reveals that BMimF provides a “naked” F − that enables the stable cycling of a symmetric Pb + PbF 2 cell with a very low overpotential of 75 mV (at 6 mA g −1 ) and 160 mV while paired with BiF 3 . XRD, AES, and SEM‐EDX reveal that, compared with the widely used TBAF·3H 2 O in THF electrolyte, BMimF in MeCN provides a more stable chemical environment for CsMnFeF 6 , a previously reported insertion‐type electrode material, by effectively limiting cation dissolution.

This study probes stack-pressure effects in ASSFIBs with BiF 3 |BaSnF 4 |Sn cells. A high 180 MPa boosts cycling by improving F − transport, mitigating oxygen-related interfacial degradation and stabilizing phase evolution of BiF 3 cathodes.

High ionic conductivity layered NH4Sn2F5 solid state electrolyte for all-solid-state fluoride-ion batteries

Optimizing the dopant concentration of solid electrolyte for room temperature fluoride ion battery applications

Fluoride-ion (F-) conductors have attracted much attention as solid electrolytes for all-solid-state fluoride-ion batteries with high energy densities surpassing those of conventional lithium-ion batteries. Ion conduction is mainly determined by the carrier amount (n) and diffusion coefficient (D), and progress is being made in understanding and controlling n. However, it is necessary to quantitatively evaluate not only D itself but also the factors that govern it. In this study, terahertz time-domain spectroscopy (THz-TDS), Fourier transform infrared spectroscopy, and first-principles calculations are used to address the effective jump attempt frequency that governs D. Phonons contributing to F- ion diffusion span a broad range of frequencies rather than a single vibrational frequency, indicating that more complex multi-phonon processes are at work. Empirical relations indicate that the mode frequency of 3THz corresponds to an activation barrier of ∼0.5eV. Phonon absorption around 5THz for LaF3 involves the F vibration with the long La-F bond length. The THz-TDS conduction increases with the phonon absorption. The loose coupling between F- and counterions to soften the lowest optically active mode increases F- conductivity.

Due to the ultrahigh theoretical energy density, no dendrite issue, and abundant resources, fluoride ion batteries (FIBs) have received a lot of attention. Regarding the issue of dissolving inorganic salt CsF in aprotic organic solvents, some anion acceptor (AA) strategies have been proposed. However, the strong binding force greatly constrains F- ion desolvation, resulting in short lifespan, low specific capacity, and poor reversibility of FIBs. Herein, we propose a concept of steric hindrance-driven closed-loop acceptor to address these problems, by using tetraphenylphosphonium chloride (Ph4PCl) with appropriate Lewis acidity to dissolve CsF and prepare a dynamic fluoride ion electrolyte based on the F-Cl exchange reaction, with a high ionic conductivity of 4.1 mS/cm at room temperature. The steric hindrance effects of chlorine and phenyl can accelerate the desolvation kinetics of F- ions. The excellent kinetics of the Ph4PCl-based electrolyte endows FIBs with long-term cycling stability, and the Sn@SnF2 symmetric cells can cycle for 500 h at 100 μA/cm2 and tolerate a critical current density as high as 1250 μA/cm2. Due to the potential dissociation ability of five-coordinated acceptor central phosphorus for fluorides and the closed-loop conversion effect of chlorine, the fluorination and defluorination reaction proceeds in a dissolution-deposition mode. The CuF2//Sn@SnF2 cell (under a high cathode loading of 4.2 mg/cm2) exhibits the highest reversible capacity up to 717.7 mAh/g and remains 316 mAh/g after 65 cycles with a small voltage polarization of only 11 mV. This work points out the novel design concept of AAs for developing high capacity and long lifespan FIBs.

Fluoride-Ion Batteries: A Review of Recent Advances and Future Opportunities

Development of fluoride-ion conductors is currently one of the main bottlenecks in realizing fluoride-ion batteries, yet many details regarding the fluoride-ion conduction mechanism hitherto remain unknown. It is well-known that La0.9Ba0.1F2.9 (LBF) has succeeded in significantly improving the ionic conductivity by introducing vacancies into the fluoride-ion sites by substituting part of La in LaF3 (P3¯c1) with Ba. Herein, a conventional solid-state electrolyte LBF (not P3¯c1 as in LaF3 but P63/mmc) was synthesized by traditional solid-state synthesis. An introduction of fluoride vacancies significantly enhanced the fluoride-ion conductivity by approximately two orders of magnitude, compared to that of LaF3, to approximately 10−6 S cm−1 at room temperature. Rietveld analysis of the temperature-dependent synchrotron x-ray diffraction patterns revealed the fluorine site-splitting from F2 along the c axis into two fluorine sites: F2 and F3. The increase in metastable fluorine sites shortens the F–F distance in the crystal structure, which could play the crucial role of fluoride-ion conduction altogether with the fluoride-ion vacancies. This work presents a proposed fluoride-ion conduction mechanism and comprehensive structural analysis, which is expected to play a key role in the understanding of the high fluoride-ion conductivity of this material.

HighlightsDeveloped perovskite fluoride solid electrolytes and electrodes via ball-milling and sintering.Achieved a high initial discharge capacity of 150 mAh g−1 at room temperature.Unveiled the charge-discharge mechanism through XRD and XPS analysis.

All-solid-state fluoride-ion batteries (FIBs) have attracted research interest due to their high energy density, which exceeds the state-of-the-art Li-ion batteries.1 Yet, the lack of a fluoride-ion conductor with both highly ionic conductive and highly stable is one of the key bottlenecks in the development of FIBs, working at low temperatures. Conventionally, La0.9Ba0.1F2.9 (LBF) is employed as a solid electrolyte in FIB. By replacing small amounts of La3+ with Ba2+, the fluoride-ion conductivity increases approximately four orders of magnitude.2 It is of therefore importance to understand the conduction mechanisms in the materials, which may allow us to shed light on the design principles towards the ideal fluoride-ion conductor. This study mainly focused on the occupancy of metastable positions of fluoride ions and anisotropic temperature factors from thermal vibrations, based on Rietveld analysis of synchrotron X-ray diffraction measurements at various temperatures from room temperature to 150°C.LBF was synthesized using a traditional solid-state synthesis at high temperatures. LaF3 and BaF2 were initially mixed via ball milling at 600 rpm for 12 h under a static argon atmosphere. The resultant powder was pelletized and heated at 1000 °C for 4 h under the flow of argon gas. The product was ground to powder and characterized by using synchrotron X-ray diffraction (XRD) at various temperatures. Impedance measurements were performed from room temperature to 150 °C using gold blocking electrodes to estimate their electrical conductivity.The structure of LBF was found to be better described in P63/mmc space group, regarding the diffraction pattern. Electrochemical impedance spectroscopy showed the Nyquist plot of the materials as a semi-circle with a spike feature at low frequencies which is a typical response of ionic conduction under the blocking electrode scheme. The conductivity is approximately 2.2×10−5 S cm−1 at 150 °C which is in accordance with values reported in literature. Analysis of the materials via Rietveld refinement enables the observation of the fluorine site splitting similar to what observed in other fluoride-ion conductors.3 The presence of the additional fluorine position results in the shortening of the fluorine-to-fluorine distance along the crystal c-axis which could form a conduction pathway of fluoride-ion. Such observation emphasizes the importance of structural elucidation in relationship with the fluoride-ion conduction in ionic conductors. Reference 1) C. Rongeat, M. A. Reddy, R. Witter and M. Fichtner, J. Phys. Chem. C, 2013, 117, 4943–4950.2) S. Breuer, S. Lunghammer, A. Kiesl and M. Wilkening, J. Mater. Sci., 2018, 53, 13669–13681.3) C. Pattanathummasid, K. Tani, K. Mori, T. Matsunaga, T. Takami, ACS Appl. Energy Mater., 2025, 8(3), 1709–1715.

High-Capacity Intercalation-Based Anodes for Solid-State Fluoride-Ion Batteries Enabled by the Substitution of Conductive Carbon by Metallic Copper

Abstract Fluoride‐ion batteries (FIBs), owing to their high theoretical energy density, wide electrochemical stability window, and the abundance of raw materials, have garnered considerable attention as promising next‐generation energy storage technologies. Solid state fluoride ion batteries (SSFIBs) are highly chemically and electrochemically stable, more stable and safer than liquid batteries. Solid‐state electrolytes (SSEs), serving as a critical component within SSFIBs, profoundly influence the electrochemical performance and safety of these batteries. This review examines recent advances in solid‐state electrolytes for fluoride‐ion batteries, with a focus on elucidating the structural characteristics, fluoride‐ion conduction mechanisms, and electrochemical performance of various electrolyte material types.

Two-dimensional W2C cathodes for fluoride-ion batteries: Achieving fast ion transport via vacancy induction

Fluoride-ion batteries, which use fluoride ions as charge carriers, are potential candidates for surpassing conventional lithium-ion batteries. Iron fluoride (FeFx, x = 3-0) is a cathode material with a high theoretical capacity of 712 mAh g-1. However, its electrochemical properties and reaction mechanism remain largely unexplored. In this study, a FeFx thin film was fabricated to investigate its discharge/charge capability and defluorination/fluorination mechanism as a cathode for all-solid-state fluoride-ion batteries. At room temperature, the FeFx cathode exhibited a reversible capacity that is 88-72% of the theoretical value at current rates of 0.1-1C. In addition, the cathode demonstrated excellent cyclability without any overvoltage increase or capacity degradation. The FeFx cathode could be discharged/charged even at a very low temperature of -30 °C, suggesting that the activation energy required for defluorination/fluorination is low. Operando X-ray absorption spectroscopy quantitatively demonstrated that the reversible discharge/charge of the FeFx cathode at room temperature was due to defluorination/fluorination between FeF3, FeF2, and Fe. The scanning electron microscopy and X-ray photoelectron spectroscopy results revealed that the electrode structure comprising uniformly distributed FeF3 fine grains approximately 10 nm in size was maintained throughout the discharge/charge cycle, suggesting that this nanometer-scale structure is related to the excellent cyclability of the FeFx cathode. Electrochemical analysis revealed that the defluorination/fluorination reaction of FeF2/Fe was rate-limited by the charge transfer process, indicating the kinetic advantage of the solid-state FeFx reaction. This study provides significant improvements in the practical properties of the fluoride-ion electrode and insights into solid-state defluorination/fluorination.

Fluoride-ion batteries (FIBs) are emerging as a potential alternative to lithium-ion batteries, offering higher energy densities, improved safety, and the use of more abundant and sustainable materials. Recent advancements in fluoride-ion technology have focused on addressing key challenges, such as the low ionic conductivity of fluoride and the development of suitable electrode materials. Researchers have made progress in creating electrolytes that stabilize fluoride ions during charging and discharging, leading to prototypes with enhanced cycling stability and energy capacity compared to earlier models. However, issues like corrosion and the need for more efficient energy storage remain significant barriers. Ongoing research is dedicated to finding novel materials that can improve conductivity, as well as to developing corrosion-resistant components that will enhance the longevity and safety of fluoride-ion batteries. Additionally, improving the overall energy efficiency and scalability of production is crucial for future commercialization. If these challenges are successfully overcome, fluoride-ion batteries could offer a transformative solution for high-energy applications, including electric vehicles, portable electronics, and large-scale grid energy storage. As research progresses, fluoride-ion batteries hold the potential to become a key technology in the quest for more sustainable, high-performance energy storage systems.

Fluoride ion batteries (FIBs), as a promising next-generation high-energy-density storage technology, have attracted significant attention. However, developing an ideal fluoride-ion electrolyte that suppresses the β-H abstraction (caused by strong Lewis-basicity F-) and electrolyte decomposition remains challenging. To address this bottleneck, we design an electrolyte system based on commercial tetrabutylammonium fluoride (TBAF) salt and 1-butyl-3-methylimidazolium tetrafluoroborate (BMImBF4) ionic liquid solvent through anion-cation coordination engineering and hard-soft-acid-base (HSAB) balance modulation, unveiling its multiscale mechanisms for mitigating interfacial parasitic reaction and enhancing metal anode stability. Experimental and theoretical analyses reveal that the soft-acid BMIm⁺ participates in the solvation structure of hard-base fluoride ions, effectively blocking the β-H elimination pathway and expanding the electrochemical window to 4.5V. The ionic conductivity of this ionic liquid based electrolyte reaches 5.0×10-3S cm-1 at 60°C even after in situ polymerization. The Cu2O cathode coupling insertion and conversion reactions can alleviate the volume deformation and capacity decay of Cu2O||Li-LiF high-voltage FIBs, with a high resting voltage (2.91V) and a high initial capacity of 589.9mAh g-1. The Cu2O||Pb-PbF2 FIBs maintain a high reversible capacity of 243.6mAh g-1 even after 800 cycles under 200mA g-1. The work establishes a novel electrolyte design paradigm for high-voltage reversible FIBs.

Abstract Fluoride ion batteries (FIBs), as a promising next‐generation high‐energy‐density storage technology, have attracted significant attention. However, developing an ideal fluoride‐ion electrolyte that suppresses the β‐H abstraction (caused by strong Lewis‐basicity F−) and electrolyte decomposition remains challenging. To address this bottleneck, we design an electrolyte system based on commercial tetrabutylammonium fluoride (TBAF) salt and 1‐butyl‐3‐methylimidazolium tetrafluoroborate (BMImBF4) ionic liquid solvent through anion–cation coordination engineering and hard–soft‐acid–base (HSAB) balance modulation, unveiling its multiscale mechanisms for mitigating interfacial parasitic reaction and enhancing metal anode stability. Experimental and theoretical analyses reveal that the soft‐acid BMIm⁺ participates in the solvation structure of hard‐base fluoride ions, effectively blocking the β‐H elimination pathway and expanding the electrochemical window to 4.5 V. The ionic conductivity of this ionic liquid based electrolyte reaches 5.0 × 10−3 S cm−1 at 60 °C even after in situ polymerization. The Cu2O cathode coupling insertion and conversion reactions can alleviate the volume deformation and capacity decay of Cu2O||Li–LiF high‐voltage FIBs, with a high resting voltage (2.91 V) and a high initial capacity of 589.9 mAh g−1. The Cu2O||Pb–PbF2 FIBs maintain a high reversible capacity of 243.6 mAh g−1 even after 800 cycles under 200 mA g−1. The work establishes a novel electrolyte design paradigm for high‐voltage reversible FIBs.

Abstract All‐solid‐state fluoride‐ion batteries (FIBs) are highly prospective candidates for next‐generation batteries due to their superior energy density and safety to lithium‐ion batteries (LIBs). Nevertheless, the full potential of FIBs remains unrealized, as practical cathode materials remain elusive. Here, a perovskite oxyfluoride SrFeO2Fx is reported to exhibit topotactic F− (de)intercalation and provide a large capacity of 350 mAh g−1 (1843 mAh cm−3) with negligibly small volume expansion (≈0.5%). The large capacity is attributed to excess F− (de)intercalation of ≈2.3 mol in SrFeO2Fx with Fe2+/Fe3+ redox and oxygen redox. Furthermore, Sr‐substituted CaFeO2Fx (Ca0.8Sr0.2FeO2Fx) has the highest capacity of 580 mAh g−1 (2595 mAh cm−3) among cathode materials using the topotactic ion (de)intercalation, including LIBs. For the first time, the possibility is presented that the utilization of ultimate anion redox associated with the ion (de)intercalation to achieve specific capacities surpassing current active materials by over twofold.

In this study, conductive, fluorine and antimony codoped tin oxide nanoparticles (FATO‐NPs) are highlighted as a possible alternative for conductive carbon additives in fluoride ion batteries, successfully circumventing oxidative side reactions. Since good cyclability with high and stable discharge capacities is achieved with both types of conductive additive at a high stack pressure of 180 MPa, it is concluded that conductive carbon is well‐suited for high‐voltage fluoride ion batteries, contrary to prior assumptions. However, FATO‐NP‐based cathodes outperform those based on conductive carbon at lower stack pressures of 50 MPa, emphasizing the importance of avoiding carbon fluorination at low stack pressures.