Conductivity Optimization of Tysonite-type La1-xBaxF3-x Solid Electrolytes for Advanced Fluoride Ion Battery

Fluorine Chemistry in Rechargeable Batteries: Challenges, Progress, and Perspectives

The renewable energy industry demands rechargeable batteries that can be manufactured at low cost using abundant resources while offering high energy density, good safety, wide operating temperature windows, and long lifespans. Utilizing fluorine chemistry to redesign battery configurations/components is considered a critical strategy to fulfill these requirements due to the natural abundance, robust bond strength, and extraordinary electronegativity of fluorine and the high free energy of fluoride formation, which enables the fluorinated components with cost effectiveness, nonflammability, and intrinsic stability. In particular, fluorinated materials and electrode|electrolyte interphases have been demonstrated to significantly affect reaction reversibility/kinetics, safety, and temperature tolerance of rechargeable batteries. However, the underlining principles governing material design and the mechanistic insights of interphases at the atomic level have been largely overlooked. This review covers a wide range of topics from the exploration of fluorine-containing electrodes, fluorinated electrolyte constituents, and other fluorinated battery components for metal-ion shuttle batteries to constructing fluoride-ion batteries, dual-ion batteries, and other new chemistries. In doing so, this review aims to provide a comprehensive understanding of the structure-property interactions, the features of fluorinated interphases, and cutting-edge techniques for elucidating the role of fluorine chemistry in rechargeable batteries. Further, we present current challenges and promising strategies for employing fluorine chemistry, aiming to advance the electrochemical performance, wide temperature operation, and safety attributes of rechargeable batteries.

Room-temperature reversible F-ion batteries based on sulfone electrolytes with a mild anion acceptor additive

Rechargeable fluoride ion batteries (FIBs) as an emerging anion shuttle system are attracting much attention due to their potential advantages in terms of energy density, cost and safety. A liquid electrolyte system enables the FIB operation at low or room temperature due to its higher ionic conductivity than that of a solid F-ion electrolyte. However, the insolubility of fluoride salts in aprotic solvents limits the development of liquid F-ion electrolytes. Although the boron-based anion acceptors (AAs) can facilitate the dissolution of F-ion salts, they are prone to lead to a tough desolvation process for F- due to strong Lewis acidity and therefore an inferior electrochemical performance. Here, a new non-boron AA (6-thioguanine) with moderate Lewis acidity is proposed to dissolve F- in the sulfone solvent. The ionic conductivity of the corresponding electrolytes reaches a level of mS cm-1 at room temperature. A model FIB coin cell is successfully operated with high conversion reaction reversibility based on the coupled defluorination/fluorination mechanism of electrodes, enabling a low overpotential of 0.36 V and a reversible capacity of 126 mA h g-1 after 40 cycles.

Reconsidering Anode Materials for Fluoride-Ion Batteries-The Unexpected Roles of Carbide Formation

Carbon is a ubiquitous additive to enhance the electrical conductivity of battery electrodes. Although carbon is generally assumed to be inert, the poor reversibility seen in some fluoride-ion battery electrodes has not been explained or systematically explored. Here, we utilize the Materials Project database to assess electrode deactivation reactions that result in the formation of a metal carbide. Specifically, we compare the theoretical potentials of MFy reduction to either the corresponding metal M or metal carbide MCx . We find that the formation of MCx is unlikely to be important in anodes that operate at modest reduction potentials, such as those made from electronegative metals like Zn, Sn, or Pb. However, in anodes that operate at extreme reduction potentials, such as alkaline earths or lanthanides, we find that formation of MCx is relevant and can emerge as a mechanism for capacity loss. Thus, side reactions of metals with carbon additives that form metal carbides possibly explain the poor reversibility of lanthanide or alkaline earth metal-based electrode materials. Finally, we highlight that the carbide formation process might be exploited for designing cheap anode systems with improved reversibility.

Challenges for fluoride superionic conductors: fundamentals, design, and applications

Electronics, which harnesses the properties of electrons, has made remarkable progress since its inception and is a cornerstone of modern society. Ionics, which exploits the properties of ions, has also had a profound impact, as demonstrated by the award of the Nobel Prize in Chemistry in 2019 for achievements related to lithium-ion batteries (LIBs). Ionic conduction in solids is the flow of carrier ions through a solid owing to an electrical or chemical bias. Some ionic materials have been studied intensively because their ionic conductivities are higher than those of liquids, even though they are solids. Among various conductive species, fluoride ions are the most promising charge carriers for fluoride-ion batteries (FIBs) as post LIBs. Increasing fluoride-ion conductivity toward the superionic conductive region at room temperature would be a breakthrough for the room-temperature operation of all-solid-state FIBs. This review focuses on fluoride-ion conductors, from the general concept of ions to the characteristics of fluoride ions. Fluoride-ion conductors are classified according to material type and form, and our current understanding, identification of problems, and future directions are discussed from experimental and theoretical physics perspectives.

Assessing ternary materials for fluoride-ion batteries

Although lithium-ion batteries have transformed energy storage, there is a need to develop battery technologies with improved performance. Fluoride-ion batteries (FIBs) may be promising alternatives in part due to their high theoretical energy density and natural elemental abundance. However, electrode materials for FIBs, particularly cathodes, have not been systematically evaluated, limiting rapid progress. Here, we evaluate ternary fluorides from the Materials Project crystal structure database to identify promising cathode materials for FIBs. Structures are further assessed based on stability and whether fluorination/defluorination occurs without unwanted disproportionation reactions. Properties are presented for pairs of fluorinated/defluorinated materials including theoretical energy densities, cost approximations, and bandgaps. We aim to supply a dataset for extracting property and structural trends of ternary fluoride materials that may aid in the discovery of next-generation battery materials.

Fast fluoride ion conduction of NH4(Mg1-xLix)F3-x and (NH4)2(Mg1-xLix)F4-x assisted by molecular cations

Aiming development of the fast anion conductors, we proposed a new material design using flexible molecular cation as a host cation, and demonstrated it with fluoride ion conduction in NH₄MgF₃ and (NH₄)₂MgF₄ based materials. Dominant fluoride ion conduction with relatively high conductivities of 4.8 × 10^-5 S cm^-1 and 8.4 × 10^-6 S cm^-1 were achieved at 323 K in (NH₄)₂(Mg_0.85Li_0.15)F_3.85 and NH₄(Mg_0.9Li_0.1)F_2.9, respectively. It is implied that the molecular cation in the host lattice can assist the anion conduction. Our findings suggest molecular cation-containing compounds can be attractive material groups for fast anion conductors.

La1–yBayF3–y Solid Solution Crystals as an Effective Solid Electrolyte: Growth and Properties

A series of nonstoichiometric La1–yBayF3–y (0 ≤ y ≤ 0.12) single crystals with a tysonite-type structure (sp. gr. P-3c1) was grown from the melt by the directional crystallization method in a fluorinating atmosphere, and some physical properties were characterized. The concentration dependence of electrical conductivity σdc(y) La1–yBayF3–y crystals was studied. The composition of the ionic conductivity maximum for this solid electrolyte was refined. It was confirmed that the maximum conductivity σmax = 8.5 × 10–5 S/cm (295 K) was observed at the composition ymax = 0.05 ± 0.01. Analysis of the electrophysical data for the group of tysonite-type solid electrolytes R1–yMyF3–y (M = Ca, Sr, Ba, Eu2+ and R = La, Ce, Pr, Nd) showed that the compositions of the maxima of their conductivity were close and amount to y = 0.03−0.05. This fact indicates a weak influence of the size effect (ionic radii R3+ and M2+) on the value of ymax for R1–yMyF3–y solid electrolytes.

Two-Phase Reaction Mechanism for Fluorination and Defluorination in Fluoride-Shuttle Batteries: A First-Principles Study

Fluoride-shuttle batteries (FSBs), which are based on fluoride-ion transfer, have attracted attention because of their high theoretical energy densities. The fluorination and defluorination reactions at the electrodes are the possible rate-determining steps in FSBs, and understanding the mechanism is important to achieve smooth charge/discharge. In this study, we discuss the thermodynamically favored pathways for the fluorination and defluorination reactions and compare the reactions through the solid-solution and two-phase-coexistent states by density functional theory (DFT) calculations. The free energies of the solid-solution and two-phase states approximate the energies calculated by DFT, and their accuracy was validated by comparison with experimental formation enthalpies and free energies. The relative formation enthalpies of typical, transition, and relativistic metal (Tl, Pb, and Bi) fluorides are well reproduced by DFT calculations within 0.1, 0.2, and 0.4 eV, respectively. We also show that the reaction pathway can be determined by comparing the formation enthalpies of the metal fluoride H, a fluorine vacancy H_V, and an interstitial fluorine defect H_I from the simple selection rule. The enthalpy relation of H_I > H > -H_V observed in all the calculations strongly suggests that fluorination and defluorination in FSB electrodes occur by a two-phase reaction. This fluorination and defluorination mechanism will be useful to clarify the rate-determining step in FSBs.

Pressure-induced ionic to mixed ionic and electronic conduction transition in solid electrolyte LaF3

LaF₃ was found to transform from pure ionic conduction to mixed ionic and electronic conduction at 15.0 GPa.

Fluoride-Ion Batteries: On the Electrochemical Stability of Nanocrystalline La0.9Ba0.1F2.9 against Metal Electrodes

Over the past years, ceramic fluorine ion conductors with high ionic conductivity have stepped into the limelight of materials research, as they may act as solid-state electrolytes in fluorine-ion batteries (FIBs). A factor of utmost importance, which has been left aside so far, is the electrochemical stability of these conductors with respect to both the voltage window and the active materials used. The compatibility with different current collector materials is important as well. In the course of this study, tysonite-type La0.9Ba0.1F2.9, which is one of the most important electrolyte in first-generation FIBs, was chosen as model substance to study its electrochemical stability against a series of metal electrodes viz. Pt, Au, Ni, Cu and Ag. To test anodic or cathodic degradation processes we carried out cyclic voltammetry (CV) measurements using a two-electrode set-up. We covered a voltage window ranging from −1 to 4 V, which is typical for FIBs, and investigated the change of the response of the CVs as a function of scan rate (2 mV/s to 0.1 V/s). It turned out that Cu is unstable in combination with La0.9Ba0.1F2.9, even before voltage was applied. The cells with Au and Pt electrodes show reactions during the CV scans; in the case of Au the irreversible changes seen in CV are accompanied by a change in color of the electrode as investigated by light microscopy. Ag and Ni electrodes seem to suffer from contact issues which, most likely, also originate from side reactions with the electrode material. The experiments show that the choice of current collectors in future FIBs will become an important topic if we are to develop long-lasting FIBs. Most likely, protecting layers between the composite electrode material and the metal current collector have to be developed to prevent any interdiffusion or electrochemical degradation processes.

Ionic conductivity and structure of M1-xPbxF2 (M = Ca, Sr, Ba) solid solutions prepared by ball milling

Nanocrystalline M1-xPbxF2 (M = Ca, Sr, Ba) solid solutions were prepared by ball milling mixtures of binary parent materials. The structure of the obtained materials was investigated by 19F MAS NMR spectroscopy and XRPD, and their ionic conductivity by impedance spectroscopy. It was found that fluorite-structured solid solutions over the whole range of compositions can be prepared by ball milling for all three systems, closing the broad miscibility gap of the CaF2-PbF2 system. The fluoride ion conductivity increased with increasing Pb content of the solid solutions, with Ba0.10Pb0.90F2 showing the highest conductivity of all samples prepared, being 1.5 orders of magnitude smaller than the one of PbSnF4. Ca1-xPbxF2 showed a fluoride ion conductivity increase which can be assigned to geometric frustration induced disorder. Ball milling of pure PbF2 revealed an increase of the fluoride ion conductivity by 2.5 orders of magnitude in the case of PbF2 containing a large amount of β-PbF2 compared to microcrystalline β-PbF2. Its fluoride ion conductivity is also 2.5 orders of magnitude larger than the fluoride ion conductivity of ball milled PbF2 consisting of a small amount of β-PbF2 but a large amount of α-PbF2 pointing to differently conducting and structured grain boundaries of α-PbF2 and β-PbF2.

Mismatch in cation size causes rapid anion dynamics in solid electrolytes: the role of the Arrhenius pre-factor

Mixed (Ba,Ca)F₂ reveals highly correlated F anion diffusion in disordered potentials landscapes.