Ionothermal Synthesis of Polyanionic Electrode Material Na3V2(PO4)2FO2 through a Topotactic Reaction

Fluorine as a Key Element in Solid-State Chemistry of Mixed Anions 3d Transition Metal-Based Materials for Electronic Properties and Energy

Mixed anion compounds containing fluorine and based on 3d transition elements represent a class of materials with significant interest in solid-state chemistry. Indeed, their highly varied chemical composition, structural diversity, and the resulting electronic properties provide a rich playground for imagining new applications in the field of energy. The anions and the chemical bonds they form with the 3d transition elements are at the heart of this review. Key parameters such as electronegativity, hardness, and polarizability are introduced and discussed to better understand the charge capacity of the anion and the bonds formed in the solid. Oxyfluorides represent the most studied family due to the size similarity of the two anions, and part of the review is dedicated to the specific synthesis of these materials by systematically adjusting the fluorine content within various structures and analyzing the electronic and electrochemical properties of these compositions. The final sections focus on materials with structures often exhibiting a two-dimensional character, where ionic blocks coexist with covalent layers, such as fluorochalcogenides, fluoropnictides, and fluorotetrelides. The compositions and structures are systematically correlated with the electronic properties.

Deep-Eutectic Solvent as a Solvent and Precursor for the Synthesis of a Carbon-Coated Na3V2(PO4)2F3-yOy Material

Deep eutectic solvents (DES) are well-known as cost-effective and environmentally friendly "designer solvents" for controlling the size and morphology of nanomaterials. In this study, we leverage DES not only as a solvent for the topochemical synthesis of Na3V2(PO4)2F3-yOy (0 ≤ y ≤ 2) but also as a precursor for a uniform and thin carbon coating. After solvothermal synthesis in a green deep eutectic solvent composed of a mixture of choline chloride, citric acid, and water (3:1:3 molar ratio), XRD refinements and FTIR, XPS, and TEM analyses confirmed the obtention of a pure Na3V2(PO4)2F3-yOy (0 ≤ y ≤ 2) phase encapsulated in an organic layer derived from a residual deep eutectic solvent. Subsequent sintering at 600 °C under an argon atmosphere produced a homogeneous nitrogen-doped carbon coating without the need for additional carbon sources. Electrochemical tests in galvanostatic conditions demonstrated that this material exhibits excellent performance in terms of capacity retention and rate capabilities, with specific capacities exceeding 110 mAh/g at 2C versus Na metal and 68 mAh/g at 10C in full cells versus hard carbon.

Particle nanosizing and coating with an ionic liquid: two routes to improve the transport properties of Na3V2(PO4)2FO2

Na₃V₂(PO₄)₂FO₂ is a promising candidate for practical use as a positive electrode material in Na-ion batteries thanks to its high voltage and excellent structural stability upon cycling. However, its limited intrinsic transport properties limit its performance at fast charge/discharge rates. In this work, two efficient approaches are presented to optimize the electrical conductivity of the electrode material: particle nanosizing and particle coating with an ionic liquid (IL). The former reveals that particle downsizing from micrometer to nanometer range improves the electronic conductivity by more than two orders of magnitude, which greatly improves the rate capability without affecting the capacity retention. The second approch dealing with an original surface modification by applying an IL coating strongly enhances the ionic mobility and offers new perspectives to improve the energy storage performance by designing the electrode materials' surface composition.

Revealing the Multi-Electron Reaction Mechanism of Na3 V2 O2 (PO4 )2 F Towards Improved Lithium Storage

Na₃ V₂ O₂ (PO₄ )₂ F (NVOPF) as an attractive electrode material has received much attention based on the one-electron reaction of V⁴⁺ /V⁵⁺ . However, the electrochemical reactions involving lower vanadium valences were not investigated till now. Herein, a composite of graphene decorated nanosheet-assembled NVOPF microflowers (NVOPF/G) was synthesized and the multi-electron reaction of NVOPF/G was conducted by controlling the operation voltage windows. The reaction mechanism, structural changes, and vanadium valences during the insertion/extraction of Li ions (from 2 to 6) were elucidated clearly by in-situ X-ray diffraction and ex-situ X-ray photoelectron spectroscopy. Theoretical computations also revealed the Li-ion locations in the structure of NaV₂ O₂ (PO₄ )₂ F. Due to the additional redox couple of V³⁺ /V⁴⁺ , NVOPF/G displayed a much higher initial capacity of 183.3 mAh g^-1 in the wider voltage window of 1.0-4.8 V than that of 2.5-4.8 V (129.3 mAh g^-1 ). Moreover, excellent Li-storage performance of NVOPF/G at a lower voltage (≤2.5 V) with the active reaction of V²⁺ /V³⁺ /V⁴⁺ was obtained for the first time, demonstrating the high potential of NVOPF/G as an anode material for Li ion storage.

Controlled Nanostructuration of Cobalt Oxyhydroxide Electrode Material for Hybrid Supercapacitors

Nanostructuration is one of the most promising strategies to develop performant electrode materials for energy storage devices, such as hybrid supercapacitors. In this work, we studied the influence of precipitation medium and the use of a series of 1-alkyl-3-methylimidazolium bromide ionic liquids for the nanostructuration of β(III) cobalt oxyhydroxides. Then, the effect of the nanostructuration and the impact of the different ionic liquids used during synthesis were investigated in terms of energy storage performances. First, we demonstrated that forward precipitation, in a cobalt-rich medium, leads to smaller particles with higher specific surface areas (SSA) and an enhanced mesoporosity. Introduction of ionic liquids (ILs) in the precipitation medium further strongly increased the specific surface area and the mesoporosity to achieve well-nanostructured materials with a very high SSA of 265 m2/g and porosity of 0.43 cm3/g. Additionally, we showed that ILs used as surfactant and template also functionalize the nanomaterial surface, leading to a beneficial synergy between the highly ionic conductive IL and the cobalt oxyhydroxide, which lowers the resistance charge transfer and improves the specific capacity. The nature of the ionic liquid had an important influence on the final electrochemical properties and the best performances were reached with the ionic liquid containing the longest alkyl chain.