Insights into Li⁺, Na⁺, and K⁺ Intercalation in Lepidocrocite-Type Layered TiO₂ Structures

A lamellar lepidocrocite-type titanate structure with ∼25% Ti⁴⁺ vacancies was recently synthesized, and it showed potential for use as an electrode in rechargeable lithium-ion batteries. In addition to lithium, we explore this material’s ability to accommodate other monovalent ions with greater natural abundance (e.g., sodium and potassium) in order to develop lower-cost alternatives to lithium-ion batteries constructed from more widely available elements. Galvanostatic discharge/charge curves for the lepidocrocite material indicate that increasing the ionic radius of the monovalent ion results in a deteriorating performance of the electrode. Using first-principles electronic structure calculations, we identify the relaxed geometries of the structure while varying the placement of the ion in the structure. We then use these geometries to compute the energy of formations. Additionally, we determine that all ions are favorable in the structure, but interlayer positions are preferred compared to vacancy positions. We also conclude that the exchange between the interlayer and vacancy positions is a process that involves the interaction between interlayer water and surface hydroxyl groups next to the titanate layer. We observe a cooperative effect between structural water and OH groups to assist alkali ions to move from the interlayer to the vacancy site. Thus, the as-synthesized lepidocrocite serves as a prototypical structure to investigate the migration mechanism of ions within a confined space along with the interaction between water molecules and the titanate framework

Anionic Ordering and Thermal Properties of FeF₃·3H₂O

Iron fluoride trihydrate can be used to prepare iron hydroxyfluoride with the hexagonal–tungsten–bronze (HTB) type structure, a potential cathode material for batteries. To understand this phase transformation, a structural description of β-FeF₃·3H₂O is first performed by means of DFT calculations and Mössbauer spectroscopy. The structure of this compound consists of infinite chains of [FeF₆]_<i>n</i> and [FeF₂(H₂O)₄]_<i>n</i>. The decomposition of FeF₃·3H₂O induces a collapse and condensation of these chains, which lead to the stabilization, under specific conditions, of a hydroxyfluoride network FeF_3–<i>x</i>(OH)_<i>x</i> with the HTB structure. The release of H₂O and HF was monitored by thermal analysis and physical characterizations during the decomposition of FeF₃·3H₂O. An average distribution of FeF₄(OH)₂ distorted octahedra in HTB-FeF_3–<i>x</i>(OH)_<i>x</i> was obtained subsequent to the thermal hydrolysis/olation of equatorial anionic positions involving F^– and H₂O. This study provides a clear understanding of the structure and thermal properties of FeF₃·3H₂O, a material that can potentially bridge the recycling of pickling sludge from the steel industry by preparing battery electrodes

Resolving and Quantifying Nanoscaled Phases in Amorphous FeF₃ by Pair Distribution Function and Mössbauer Spectroscopy

Probing the atomic structure of materials displaying a lack of long-range order has been a continuous challenge for the material science’s community. X-ray amorphous FeF₃ has been shown to be a promising electrode material in Li and Na ion batteries. Providing structural information on this class of compounds is therefore of interest as it can help rationalize the material’s properties and further enabled its optimization. Herein, we used the pair distribution function and Mössbauer spectroscopy to provide unique insights into the atomic structure of amorphous FeF₃. The results showed that amorphous FeF₃ contained two phases built from corner-sharing of FeF₆ octahedra. According to X-ray diffraction data, the PDF was successfully modeled based on two structural models related to the distorted ReO₃ and the hexagonal-tungsten-bronze networks of FeF₃. The lack of long-range order shown by conventional XRD data and PDF analysis was shown to arise mostly from disorder. This study provides detailed atomic structure with corresponding spectroscopic signature of amorphous phases. Quantitative analysis of both techniques indicated similar trends. This showed that our approach can be employed to determine the structure of other complex materials

Solid Fluoride Electrolytes and Their Composite with Carbon: Issues and Challenges for Rechargeable Solid State Fluoride-Ion Batteries

Solid-state batteries relying on fluoride-ion shuttle are still at their early stage of development. Assessing the fluoride solid electrolyte’s electrochemical stability and its conduction properties in a mixture with carbon, as well as the possible interaction of fluoride-ion with carbon both during the electrode preparation and upon electrochemical reactions, are mandatory to enable future practical applications. Here, we discuss these points using LaF₃ doped with BaF₂ (La_0.95Ba_0.05F_2.95, LBF) as a benchmark solid fluoride electrolyte. We establish that lithium may be used as a pseudoreference electrode to assess the electrochemical stability window of LBF and support the experiment with thermodynamic calculations. We demonstrate the chemical compatibility of LBF with carbon upon ball-milling and investigate the electrical conductivity of the formed LBF-C composite. We use a LBF|LBF-C|LBF cell (in this configuration, LBF serves as electronically blocking electrode) to assess the ionic conductivity of the LBF-C composite. The results confirm that both electronic and ionic percolations are ensured within the LBF-C composite despite a noticeable decrease of the ionic conductivity. Finally, we use a Li|LBF|LBF-C cell to evaluate the electrochemical fluorination of the carbon in the LBF-C composite. Our results suggest an electrochemical activity of carbon with fluoride ions. The possible interactions of carbon with fluorides to form insulating carbon fluoride (CF_<i>x</i>) must be considered when determining the operating voltage of FIBs

Structural and Morphological Description of Sn/SnO_<i>x</i> Core–Shell Nanoparticles Synthesized and Isolated from Ionic Liquid

The potential application of high capacity Sn-based electrode materials for energy storage, particularly in rechargeable batteries, has led to extensive research activities. In this scope, the development of an innovative synthesis route allowing to downsize particles to the nanoscale is of particular interest owing to the ability of such nanomaterial to better accommodate volume changes upon electrochemical reactions. Here, we report on the use of room temperature ionic liquid (i.e., [EMIm⁺]­[TFSI^–]) as solvent, template, and stabilizer for Sn-based nanoparticles. In such a media, we observed, using Cryo-TEM, that pure Sn nanoparticles can be stabilized. Further washing steps are, however, mandatory to remove residual ionic liquid. It is shown that the washing steps are accompanied by the partial oxidation of the surface, leading to a core–shell structured Sn/SnO_<i>x</i> composite. To understand the structural features of such a complex architecture, HRTEM, Mössbauer spectroscopy, and the pair distribution function were employed to reveal a crystallized β-Sn core and a SnO and SnO₂ amorphous shell. The proportion of oxidized phases increases with the final washing step with water, which appeared necessary to remove not only salts but also the final surface impurities made of the cationic moieties of the ionic liquid. This work highlights the strong oxidation reactivity of Sn-based nanoparticles, which needs to be taken into account when evaluating their electrochemical properties

Tailoring the Composition of a Mixed Anion Iron-Based Fluoride Compound: Evidence for Anionic Vacancy and Electrochemical Performance in Lithium Cells

Microwave-assisted synthesis allows stabilizing Fe-based fluoride compounds with hexagonal tungsten bronze (HTB) network. The determination of the chemical composition, i.e., FeF_2.2(OH)_0.8·(H₂O)_0.33, revealed a significant deviation from the pure fluoride composition, with a high content of OH groups substituting fluoride ions. Rietveld refinement of the X-ray diffraction data and Mössbauer spectroscopy showed that the partial OH/F substitution impact on the structure (interatomic distances, angles, and so on) and the local environment of iron (isomer shift and quadrupole splitting distribution). The thermal behavior of the hydroxyfluoride compound has been thoroughly investigated. From room temperature to 350 °C under Ar flow, the HTB-type structure remains stable without any fluorine loss and only water departure. At <i>T</i> > 350 °C, the structure started to collapse with a partitioning of anions leading to α-FeF₃ and α-Fe₂O₃. Within 200 °C ≤ <i>T</i> ≤ 350 °C, the chemical composition can be tuned with different contents of OH^–/O^2– and structural water. By an adequate thermal treatment, it has been shown that anionic vacancies formed by dehydroxylation reaction could be stabilized within the HTB network yielding a compound containing three different anions, i.e., FeF_2.2(OH)_{0.8–<i>x</i>}O_<i>x</i>/2□_<i>x</i>/2. XRD Rietveld analysis, atomic pair distribution function, and Mössbauer spectroscopy confirmed the formation of under-coordinated iron FeX₅□₁ (X = O^2–, F^–, and OH^–) atoms. Different compositions have been prepared by thermal treatment at <i>T</i> ≤ 350 °C and their electrochemical properties evaluated in lithium cell. Structural water seems to block the diffusion of lithium within the hexagonal cavities. Increasing the content of anionic vacancies significantly improves the reversible capacity emphasizing a peculiar role on electrochemical properties. Pair distribution functions obtained on lithiated and delithiated samples indicated that the HTB network was maintained (in the 2–4.2 V voltage range) during the intercalation processes

Layered Lepidocrocite Type Structure Isolated by Revisiting the Sol–Gel Chemistry of Anatase TiO₂: A New Anode Material for Batteries

Searches for new electrode materials for batteries must take into account financial and environmental costs to be useful in practical devices. The sol–gel chemistry has been widely used to design and implement new concepts for the emergence of advanced materials such as hydride organic–inorganic composites. Here, we show that the simple reaction system including titanium alkoxide and water can be used to stabilize a new class of electrode materials. By investigating the crystallization path of anatase TiO₂, an X-ray amorphous intermediate phase has been identified whose local structure probed by the pair distribution function, ¹H solid-state NMR and density functional theory (DFT) calculations, consists of a layered type structure as found in the lepidocrocite. This phase presents the following general formula Ti_2–<i>x</i>□_<i>x</i>O_{4–4<i>x</i>}(OH)_4<i>x</i>·<i>n</i>H₂O (<i>x</i> ∼ 0.5) where the substitution of oxide by hydroxide anions leads to the formation of titanium vacancies (□) and H₂O molecules are located in interlayers. Solid-state ¹H NMR has enabled us to characterize three main hydroxide environments, Ti□–OH, Ti₂□₂–OH, and Ti₃□–OH, and layered H₂O molecules. The electrochemical properties of this phase were investigated  vs lithium and were shown to be very promising with reversible capacities of around 200 mAh·g^–1 and an operating voltage of 1.55 V. We further showed that the lithium intercalation proceeds via a solid-solution mechanism. ⁷Li solid-state NMR and DFT calculations allowed us to identify lithium host sites that are located at the titanium vacancies and interlayer space with lithium being solvated by structural water molecules. The easy fabrication, the absence of lithium, easier recycling, and the encouraging properties make this class of materials very attractive for competitive electrodes for batteries. We thus demonstrate that revisiting an “old” chemistry with advanced characterization tools allows one to discover new materials of technological relevance

High Substitution Rate in TiO₂ Anatase Nanoparticles with Cationic Vacancies for Fast Lithium Storage

Doping is generally used to tune and enhance the properties of metal oxides. However, their chemical composition cannot be readily modified beyond low dopant amounts without disrupting the crystalline atomic structure. In the case of anatase TiO₂, we introduce a new solution-based chemical route allowing the composition to be significantly modified, substituting the divalent O^2– anions by monovalent F^– and OH^– anions resulting in the formation of cationic Ti⁴⁺ vacancies (□) whose concentration can be controlled by the reaction temperature. The resulting polyanionic anatase has the general composition Ti_{1–<i>x</i>–<i>y</i>}□_{<i>x</i>+<i>y</i>}O_{2–4(<i>x</i>+<i>y</i>)}F_4<i>x</i>(OH)_4<i>y</i>, reaching vacancy concentrations of up to 22%, i.e., Ti_0.78□_0.22O_1.12F_0.4(OH)_0.48. Solid-state ¹⁹F NMR spectroscopy reveals that fluoride ions can accommodate up to three different environments, depending on Ti and vacancies (i.e. Ti₃-F, Ti₂□₁-F, and Ti₁□₂-F), with a preferential location close to vacancies. DFT calculations further confirm the fluoride/vacancy ordering. When its characteristics were evaluated as an electrode for reversible Li-ion storage, the material shows a modified lithium reaction mechanism, which has been rationalized by the occurrence of cationic vacancies acting as additional lithium hosting sites within the anatase framework. Finally, the material shows a fast discharging/charging behavior, compared to TiO₂, highlighting the benefits of the structural modifications and paving the way for the design of advanced electrode materials, based on a defect mediated mechanism