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.

Two-Component Rare-Earth Fluoride Materials with Negative Thermal Expansion Based on a Phase Transition-Type Mechanism in 50 RF3-R'F3 (R = La-Lu) Systems

The formation of materials with negative thermal expansion (NTE) based on a phase transition-type mechanism (NTE-II) in 50 T-x (temperature-composition) RF3-R'F3 (R = La-Lu) systems out of 105 possible is predicted. The components of these systems are "mother" RF3 compounds (R = Pm, Sm, Eu, and Gd) with polymorphic transformations (PolTrs), which occur during heating between the main structural types of RF3: β-(β-YF3) → t-(mineral tysonite LaF3). The PolTr is characterized by a density anomaly: the formula volume (Vform) of the low-temperature modification (Vβ-) is higher than that of the high-temperature modification (Vt-) by a giant value (up to 4.7%). In RF3-R'F3 systems, isomorphic substitutions chemically modify RF3 by forming R1-xR'xF3solid solutions (ss) based on both modifications. A two-phase composite (β-ss + t-ss) is a two-component NTE-II material with adjustable parameters. The prospects of using the material are estimated using the parameter of the average volume change (ΔV/Vav). The Vav at a fixed gross composition of a system is determined by the β-ss and t-ss decay (synthesis) curves and the temperature T. The regulation of ΔV/Vav is achieved by changing T within a "window ΔT". The available ΔT values are determined using phase diagrams. A chemical classification (ChCl) translates the search for NTE-II materials from 15 RF3 into an array of 105 RF3-R'F3 systems. Phase diagrams are divided into 10 types of systems (TypeSs), in four of which NTE-II materials are formed. The tables of the systems that comprise these TypeSs are presented. The position of Ttrans of the PolTr on the T scale for a short quasi-system (QS) "from PmF3 to TbF3" determines the interval of the ΔTtrans offset achievable in the RF3-R'F3 systems: from -148 to 1186 ± 10 °C. NTE-II fluoride materials exceed known NTE-II materials by almost three times in this parameter. Equilibrium in RF3-R'F3 systems is established quickly. The number of qualitatively different two-component fluoride materials with the giant NTE-II can be increased by more than ten times compared to RF3 with NTE-II.

Topotactic fluorination of intermetallics as an efficient route towards quantum materials

Intermetallics represent an important family of compounds, in which insertion of light elements (H, B, C, N) has been widely explored for decades to synthesize novel phases and promote functional materials such as permanent magnets or magnetocalorics. Fluorine insertion, however, has remained elusive so far since the strong reactivity of this atypical element, the most electronegative one, tends to produce the chemical decomposition of these systems. Here, we introduce a topochemical method to intercalate fluorine atoms into intermetallics, using perfluorocarbon reactant with covalent C-F bonds. We demonstrate the potential of this approach with the synthesis of non-stoichiometric mixed anion (Si-F) LaFeSiF_x single-crystals, which are further shown to host FeSi-based superconductivity. Fluorine topochemistry on intermetallics is thus proven to be an effective route to provide functional materials where the coexistence of ionic and metallo-covalent blocks, and their interactions through inductive effects, is at the root of their functional properties.

Insertion of light elements in intermetallics has been explored to synthesize functional materials. Here the authors report topotactic intercalation of fluorine atoms into intermetallics using a perfluorocarbon reactant with covalent C-F bonds to obtain quantum materials.

A Fluoride-Ion-Conducting Solid Electrolyte with Both High Conductivity and Excellent Electrochemical Stability

Solid-state fluoride-ion batteries (FIBs) circumvent multiple formidable bottlenecks of lithium-ion batteries, but their overall performance remains inferior due to the absence of appropriate solid electrolytes. Presently the conductivity of most solid electrolytes for FIBs is too low to enable room-temperature cycling, while the few sufficiently conductive ones only allow for very low discharge voltages because of the narrow electrochemical stability window (ESW). Here, high room-temperature conductivity and a decent ESW are simultaneously achieved by designing a solid electrolyte CsPb_0.9 K_0.1 F_2.9 . Its room-temperature conductivity is 1.23 × 10^-3  S cm^-1 , comparable to the most conductive system reported so far (PbSnF₄ , 5.44 × 10^-4 -1.6 × 10^-3  S cm^-1 ), but the ESW is several times broader. With these appealing characteristics simultaneously achieved in the solid electrolyte, a cell with much higher voltages than other room-temperature-operable solid-state FIBs in literature is successfully constructed, and stably cycled at 25 °C for 4581 h without considerable capacity fade.

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.

Growth from the Melt and Properties Investigation of ScF3 Single Crystals

ScF3 optical quality bulk crystals of the ReO3 structure type (space group P m 3 ¯ m , a = 4.01401(3) Å) have been grown from the melt by Bridgman technique, in fluorinating atmosphere for the first time. Aiming to substantially reduce vaporization losses during the growth process graphite crucibles were designed. The crystal quality, optical, mechanical, thermal and electrophysical properties were studied. Novel ScF3 crystals refer to the low-refractive-index (nD = 1.400(1)) optical materials with high transparency in the visible and IR spectral region up to 8.7 µm. The Vickers hardness of ScF3 (HV ~ 2.6 GPa) is substantially higher than that of CaF2 and LaF3 crystals. ScF3 crystals possess unique high thermal conductivity (k = 9.6 Wm−1К−1 at 300 K) and low ionic conductivity (σ = 4 × 10−8 Scm−1 at 673 К) cause to the structural defects in the fluorine sublattice.

Synergistic catalysis of carbon-partitioned LaF3–BaF2 composites for the coupling of CH4 with CHF3 to VDF

Catalytic coupling of CH₄ with potent greenhouse gas CHF₃ to vinylidene fluoride (VDF) was investigated over composite catalysts.

Use of inorganic fluorinated materials in lithium batteries and in energy conversion systems

After a review on the wide variety of inorganic fluorinated components in modern technologies, in particular for energy conversion/storage systems, the use of fluorinated carbons as electrodes for primary lithium batteries will be highlighted; in particular conventional graphite fluorides will be compared to recently investigated fluorinated carbon nanoparticles (F-CNPs) prepared from electrochemical reduction of molten carbonates.

Synthesis of Fast Fluoride-Ion-Conductive Fluorite-Type Ba1- xSb xF2+ x (0.1 ≤ x ≤ 0.4): A Potential Solid Electrolyte for Fluoride-Ion Batteries

Toward the development of high-performance solid electrolytes for fluoride-ion batteries, fluorite-type nanostructured solid solutions of Ba_{1- x}Sb _xF_{2+ x} ( x ≤ 0.4) were synthesized by high-energy ball-milling method. Substitution of divalent Ba²⁺ by trivalent Sb³⁺ leads to an increase in interstitial fluoride-ion concentration, which enhances the ionic conductivity of the Ba_{1- x}Sb _xF_{2+ x} (0.1 ≤ x ≤ 0.4) system. Total ionic conductivities of 4.4 × 10^-4 and 3.9 × 10^-4 S cm^-1 were obtained for Ba_0.7Sb_0.3F_2.3 and Ba_0.6Sb_0.4F_2.4 compositions at 160 °C, respectively. In comparison to isostructural Ba_0.3La_0.7F_2.3, the ionic conductivity of Ba_0.7Sb_0.3F_2.3 is significantly higher, which is attributed to the presence of an electron lone pair on Sb³⁺. Introduction of such lone pairs seems to increase fluoride-ion mobility in solid solutions. In addition, Ba_0.7Sb_0.3F_2.3 was tested as a cathode material against Ce and Zn anode using La_0.9Ba_0.1F_2.9 as the electrolyte. Ba_0.3Sb_0.7F_2.3/La_0.9Ba_0.1F_2.9/Ce cell showed high discharge and charge capacities of 301 and 170 mA h g^-1, respectively, in the first cycle at 150 °C.

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

Use of lithium ion batteries is currently the method of choice when it comes to local stationary storage of electrical energy. In the search for an alternative system, fluoride ion batteries (FIBs) emerge as a candidate due to their high theoretical capacity, and no lithium is needed for its operation. To improve the cycling performance and lower the working temperature of a solid-state battery, one of the critical components is the electrolyte, which needs advanced performance. This paper aims at developing an electrolyte with enhanced ionic conductivity for fluoride ions, to be used in a FIB. Tysonite La_1-xBa_xF_3-x (0 ≤ x ≤ 0.15) solid solutions were synthesized by a facile wet chemical method, and its ionic conductivity was analyzed using electrochemical impedance spectroscopy. A composition study shows that the conductivity reaches a maximum of 1.26 × 10^-4 S·cm^-1 at 60 °C for the La_0.95Ba_0.05F_2.95 pellet sintered at 800 °C for 20 h, which is 1 order of magnitude higher than that for the as-prepared pellet and 2 times higher than the conductivity of sintered ball-milled batches. The reason for this dramatic increment is the more efficient decrement of grain boundary resistance upon sintering. Morphological, chemical, and structural characterizations of solid electrolytes were studied by X-ray diffraction, scanning electron microscopy , energy dispersive X-ray spectroscopy, physisorption by the Brunauer-Emmett-Teller method, and transmission electron microscopy. Electrochemical testing was carried out for the FIB cell using La_0.95Ba_0.05F_2.95 as electrolyte due to its highest conductivity among the compositions, Ce as anode, and BiF₃ as a cathode. The cycling performance was found to be considerably improved when compared to our earlier work, which used the ball-milled electrolyte.

The key role of the composition and structural features in fluoride ion conductivity in tysonite Ce1−xSrxF3−x solid solutions

Evolution with x of RT ionic conductivity of RE_1−xAE_xF_3−x showing that Ce_1−xSr_xF_3−x is the best F⁻ tysonite conductor is discussed.