Hierarchical Mesoporous Iron Fluoride with Superior Rate Performance for Lithium-Ion Batteries

Ultrafine FeF3·0.33H2O Nanocrystal-Doped Graphene Aerogel Cathode Materials for Advanced Lithium-Ion Batteries

FeF₃ has been extensively studied as an alternative positive material owing to its superior specific capacity and low cost, but the low conductivity, large volume variation, and slow kinetics seriously hinder its commercialization. Here, we propose the in situ growth of ultrafine FeF₃·0.33H₂O NPs on a three-dimensional reduced graphene oxide (3D RGO) aerogel with abundant pores by a facile freeze drying process followed by thermal annealing and fluorination. Within the FeF₃·0.33H₂O/RGO composites, the three-dimensional (3D) RGO aerogel and hierarchical porous structure ensure rapid diffusion of electrons/ions within the cathode, enabling good reversibility of FeF₃. Benefiting from these advantages, a superior cycle behavior of 232 mAh g^-1 under 0.1C over 100 cycles as well as outstanding rate performance is achieved. These results provide a promising approach for advanced cathode materials for Li-ion batteries.

Cr-Doped Fe1-xCrxF3·0.33H2O Nanomaterials as Cathode Materials for Sodium-Ion Batteries

Due to the high theoretical specific capacity and low cost, FeF₃·0.33H₂O has become one of the potential choices of cathode materials for sodium-ion batteries. However, the poor intrinsic conductivity limits its practical applications. Herein, the atomic substitution is used to improve its intrinsic conductivity. The first-principles calculation results show that Cr³⁺ doping can reduce the band gap of FeF₃·0.33H₂O to improve its intrinsic conductivity. The discharge specific capacity of Fe_0.95Cr_0.05F₃·0.33H₂O with a narrowest band gap is 194.02 mA h/g at 0.1 C within the range of 1.4-4.0 V, which is higher than that of FeF₃·0.33H₂O (136.47 mA h/g). Using the electrochemical impedance spectroscopy and galvanostatic intermittent titration technique tests, it is found that R_ct of Fe_0.95Cr_0.05F₃·0.33H₂O is reduced and D_Na⁺ is almost unchanged, as compared to FeF₃·0.33H₂O.

Hierarchical Mesoporous Iron Fluoride and Reduced Graphene Oxide Nanocomposite as Cathode Materials for High-Performance Sodium-Ion Batteries

Sodium-ion batteries have been considered as one of ideal power sources for energy storage system. However, the choice of cathode material with good cycling stability and high capacity is limited. Herein, a nanocomposite of hierarchical mesoporous iron fluoride and reduced graphene oxide is prepared by an in situ approach. The as-prepared nanocomposite exhibits remarkably high discharge specific capacity of 227.5 mAh/g at 0.1C. Specifically, the discharge specific capacity of the sample still remains 87.5 mAh/g at a high rate of 15C after the 100th cycle. The electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration technique (GITT) measurements show that the addition of reduced graphene oxide can effectively reduce the charge transfer resistance and enhance the Na⁺ diffusion rate in the FeF₃·0.33H₂O nanoparticles. The structural changes of FeF₃·0.33H₂O is further investigated by ex-situ XRD, XPS, and ex situ high-resolution transmission electron microscopy.

Mechanism Study of Carbon Coating Effects on Conversion-Type Anode Materials in Lithium-Ion Batteries: Case Study of ZnMn2O4 and ZnO-MnO Composites

The carbon coating strategy is intensively used in the modification of conversion-type anode materials to improve their cycling stability and rate capability. Thus, it is necessary to elucidate the modification mechanism induced by carbon coating. For this purpose, bare ZnMn₂O₄, carbon-derivative-coated ZnMn₂O₄, and carbon-coated ZnO-MnO composite materials have been synthesized and investigated in-depth. Herein, high-temperature synchrotron radiation diffraction is used to monitor the phase transition from ZnMn₂O₄ to ZnO-MnO composite during the carbonization process. The electrochemical performance has been evaluated by cyclic voltammetry, galvanostatic cycling, and electrochemical impedance spectroscopy. The carbon- and carbon-derivative-coated samples display well-improved cycling stability in terms of suppressed electrode polarization, a moderate increase in resistance, and slight capacity variation. The influence of carbon coating on the intrinsic conversion process is investigated by ex situ X-ray absorption spectroscopy, which reveals the evolution of Zn and Mn oxidation states. This result confirms that the strong capacity variation of the bare ZnMn₂O₄ is induced not only by the reversible charge storage in the solid electrolyte interphase but also by the phase evolution of active materials. Carbon coating is an effective method to prevent the additional oxidation of MnO to Mn₃O₄, which leads to a stabilization of the main conversion reaction.

3D Starfish-Like FeOF on Graphene Sheets: Engineered Synthesis and Lithium Storage Performance

To address the problems associated with poor conductivity and large volume variation in practical applications as a conversion cathode, engineering of hierarchical nanostructured FeOF coupled with conductive decoration is highly desired, yet rarely reported. Herein, 3D starfish-like FeOF on reduced graphene oxide sheets (FeOF/rGO) is successfully prepared, for the first time, through a combination of solvothermal reaction, self-assembly, and thermal reduction. Integrating the structural features of the 3D hierarchical nanostructure, which favorably shorten the path for electron/ion transport and alleviate volumetric changes, with those of graphene wrapping, which can further enhance the electrical conductivity and maintain the structural stability of the electrode, the as-prepared FeOF/rGO composite exhibits a superior lithium-storage performance, including a high reversible capacity (424.5 mA h^-1  g^-1 at 50 mA g^-1 ), excellent stability (0.016 % capacity decay per cycle during 180 cycles), and remarkable rate capability (275.8 mA h^-1  g^-1 at 2000 mA g^-1 ).

Mn-Doped Fe1- xMn xF3·0.33H2O/C Cathodes for Li-Ion Batteries: First-Principles Calculations and Experimental Study

Increasing attention has been paid on iron fluoride as an alternative cathode material for Li-ion batteries (LIBs) owing to its high energy density and low cost. However, the poor electric conductivity and low diffusivity for Li-ions set great challenges for iron fluoride to be used in practical LIBs. Here, we employ first-principles calculations to probe the influence of Mn-doping on the crystal structure and electronic structure of FeF₃·0.33H₂O. The calculated results suggest that Mn-doping can enlarge the hexagonal cavity and reduce the band gap of FeF₃·0.33H₂O as well as improve its intrinsic conductivity. Furthermore, Fe_{1- x}Mn _xF₃·0.33H₂O/C ( x = 0, 0.06, 0.08, and 0.10) nanocomposites were successfully fabricated by a hydrothermal method and ball-milling. Owing to the Mn-doping effect combined with highly conductive acetylene black (AB) modification, the typical Fe_0.92Mn_0.08F₃·0.33H₂O/C composite exhibits a high discharge capacity of 180 mA h g^-1 at 50 mA g^-1 after 100 cycles and delivers excellent cycling stability as well as good rate capability.

Synthesis and characterization of MgF2–CoF2 binary fluorides. Influence of the treatment atmosphere and temperature on the structure and surface properties

Research was carried out on the incorporation of divalent cobalt cations into the crystalline structure of MgF₂ to form Mg_xCo_1−xF₂ binary fluorides, which had not been investigated before. The above fluorides were obtained by the precipitation from aqueous solution of magnesium and cobalt nitrates with ammonium fluoride. Binary fluorides containing 0.6, 7.5 and 37.7 mol% CoF₂ were prepared. The effects of treatment temperature (300, 400 °C) and atmosphere (oxidizing or reducing) on the structure (XRD, TPR-H₂, UV-Vis), texture (low-temperature N₂ adsorption), surface composition (XPS) and surface acidity (NH₃-TPD) of the binary fluorides were determined. It has been found that in Mg_xCo_1−xF₂ an isomorphic substitution occurs of Mg²⁺ cations by Co²⁺ cations which results in the formation of a rutile-type solid solution. The obtained binary fluorides are characterized by a mesoporous structure and relatively large surface area. It has been found that thermal treatment of the binary fluorides in oxidizing conditions results in the oxidation of CoF₂ to Co₃O₄ even at 300 °C; therefore it is not possible to obtain pure Mg_xCo_1−xF₂ binary fluorides in the presence of air. The preparation of the latter requires reducing conditions, namely thermal treatment of dry precipitate at 300 °C in an atmosphere of hydrogen. If the treatment is conducted at a higher temperature (400 °C), CoF₂ undergoes a partial reduction to metallic cobalt. An XPS study has shown the presence of hydroxyl groups in the investigated samples. However, these are solely surface groups because their presence was not detected by XRD measurements. The binary fluorides obtained by our method are characterized by a very narrow optical energy gap (5.31–3.50 eV), considerably narrower than that recorded for bulk fluorides. Measurements of temperature-programmed desorption of ammonia have shown that the incorporation of cobalt cations into the crystal structure of MgF₂ results in a decrease in the surface acidity of the binary fluorides.

Results of the first research on structural and surface properties of binary fluorides MgF₂–CoF₂ are presented.