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

The Effect of Ni Doping on FeOF Cathode Material for High-Performance Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have emerged as a promising alternative to lithium-ion batteries for large-scale energy storage systems due to the abundance and low price of sodium. Until recently, the low theoretical capacities of intercalation-type cathodes less than 250 mAh g-1 have limited the energy density of SIBs. On the other hand, iron oxyfluoride (FeOF) has a high theoretical capacity of ≈885 mAh g-1 as a conversion-type cathode material for SIBs. However, FeOF suffers from poor cycling stability, rate capability, and low initial Coulombic efficiency caused by its low electrical conductivity and slow ionic diffusion kinetics. To solve these problems, doping aliovalent Ni2+ on FeOF electrodes is attempted to improve the electronic conductivity without using a carbon matrix. The ionic conductivity of FeOF is also enhanced due to the formation of oxygen defects in the FeOF crystal structure. The FeOF-Ni1 electrode shows an excellent cycling performance with a reversible discharge capacity of 450.4 mAh g-1 at 100 mAh g-1 after 100 cycles with a fading rate of 0.20% per cycle. In addition, the FeOF-Ni1//hard carbon full cell exhibited a high energy density of 876.9 Wh kg-1 cathode with a good cycling stability.

Fluorinated Materials as Positive Electrodes for Li- and Na-Ion Batteries

Fluorine is known to be a key element for various components of batteries since current electrolytes rely on Li-ion salts having fluorinated ions and electrode binders are mainly based on fluorinated polymers. Metal fluorides or mixed anion metal fluorides (mainly oxyfluorides) have also gained a substantial interest as active materials for the electrode redox reactions. In this review, metal fluorides for cathodes are considered; they are listed according to the dimensionality of the metal fluoride subnetwork. The synthesis conditions and the crystal structures are described; the electrochemical properties are briefly indicated, and the nature of the electron transport agent is noted. We stress the crucial importance of the elaboration processes to induce the presence of cation disorders, of anion substitutions (mainly F-/O2- or F-/OH-) or vacancies. Finally, we show that an accurate structural characterization is a key step to enable enhanced material performances to overcome several lasting roadblocks, namely the large irreversible capacity and poor energy efficiency that are frequently encountered.

Li/Na Ion Storage Performance of a FeOF Nano Rod with Controllable Morphology

Although the conversion material iron oxyfluoride (FeOF) possesses a high theoretical specific capacity as a cathode material for Li/Na ion batteries, its poor rate and cycling performances, caused mainly by sluggish (Li+/Na+) reaction kinetics, restrict its practical application. Herein, FeOF with high purity, a fusiform nanorod shape and high crystallinity is prepared through a facile chemical solution reaction. The electrochemical measurements show that the present FeOF exhibits high capacity and good cycling stability as a cathode material for Li-ion batteries. Capacities of 301, 274, 249, 222, and 194 mAh/g at stepwise current densities of 20, 50, 100, 200, and 400 mA/g are achieved, respectively. Additionally, the capacity at 100 mA/g retains 123 mAh/g after 140 cycles. Meanwhile, as a cathode material for Na ion battery, it delivers discharge capacities of 185, 167, 151, 134 and 115 mAh/g at stepwise current densities of 20, 50, 100, 200, and 400 mA/g, respectively. A discharge capacity of 83 mAh/g at 100 mA/g is achieved after 140 cycles. The excellent lithium/sodium-storage performance of the present FeOF material is ascribed to its unique nanostructure.

Nanostructured Iron Fluoride Derived from Fe-Based Metal-Organic Framework for Lithium Ion Battery Cathodes

A comprehensive strategy for the morphological control of octahedral and spindle Fe-based metal-organic frameworks (Fe-MOFs) via microwave-assisted adjustment is proposed in this research. Afterward, in situ copyrolysis under N₂ atmosphere contributes to the fabrication of two shape-maintained FeF₃·0.33H₂O nanostructures (named O-FeF₃·0.33H₂O and S-FeF₃·0.33H₂O, respectively) with confined hierarchical porosity and graphitized carbon skeleton. The lithium storage performances for the MOF-derived octahedral O-FeF₃·0.33H₂O and spindle S-FeF₃·0.33H₂O composites are investigated, and the prospective lithium storage mechanism is discussed. As a result, the main product of the porous O-FeF₃·0.33H₂O structure is found to be a promising cathode material for lithium ion batteries owing to its advantageous electrochemical capability. Even after being cycled over 1000 times at 2 C (1 C = 237 mAh g^-1), the capacity attenuation rate of the as-prepared O-FeF₃·0.33H₂O electrode is as low as 0.039% per cycle. The combination of proper octahedral morphology and highly graphitized carbon modification can not only enhance the conductivity of the cathode but also promote the diffusion of Li⁺ effectively. The remarkable performance of octahedral O-FeF₃·0.33H₂O can be confirmed by the Li-ion diffusion coefficient (D_Li⁺) calculation analysis and kinetics analysis of lithium storage behavior.

FeOF/TiO2 Hetero-Nanostructures for High-Areal-Capacity Fluoride Cathodes

Iron fluoride compounds offer an exciting pathway toward low-cost and high-capacity conversion-type lithium-ion battery (LIB) cathodes. However, due to the sluggishness of the electronic and ionic transport in iron fluorides, mass loadings of active materials in previous studies are typically less than 2.5 mg cm^-2, which is too low for practical applications. Herein, we improve the charge transport in fluoride electrodes at both nano- and mesoscales to enable high-mass-loading fluoride electrodes. At the nanoscale, we prepare electronically conducting Li_xTiO₂ composites with FeOF nanoparticles to reduce electron transport distance to 5-10 nm, which is one of the shortest among reports. At the mesoscale, we design a percolating three-dimensional porous carbon nanotube (CNT) network to enable fast pathways for both electrons and ions. The resulting spongelike material, FeOF/TiO₂@CNT, substantially enhances the kinetics of the conversion reaction in FeOF, boosts extra lithium storage capacity, and reduces the voltage hysteresis. Steady cycling over 300 cycles is achieved at a high mass loading of 8.7 mg cm^-2 (FeOF/TiO₂) (1.74 mAh cm^-2). Such areal capacity of lithium storage is significantly higher than previously reported iron fluorides-based structures, a significant step forward toward the development of low-cost metal fluoride electrodes.