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

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.