Mesoporous Iron Trifluoride Microspheres as Cathode Materials for Li-ion Batteries

Metal Fluoride Cathode Materials for Lithium Rechargeable Batteries: Focus on Iron Fluorides

Exploring prospective rechargeable batteries with high energy densities is urgently needed on a worldwide scale to address the needs of the large-scale electric vehicle market. Conversion-type metal fluorides (MFs) are attractive cathodes for next-generation rechargeable batteries because of their high theoretical potential and capacities and provide new perspectives for developing novel battery systems that satisfy energy density requirements. However, some critical issues, such as high voltage hysteresis and poor cycling stability must be solved to further enhance MF cathode materials. In this review, the recent advances in mechanisms focused on FeF₃ cathodes under lithiation/delithiation processes are discussed in detail. Then, the classifications and advantages of various synthesis methods to prepare MF-based materials are first minutely discussed. Moreover, the performance attenuation mechanisms of MFs and the effort in the development of mitigation strategies are comprehensively reviewed. Finally, prospects for the current obstacles and possible research directions, with the aim to provide some inspiration for the development of MF cathode-based batteries are presented.

Intercalated Water and Organic Molecules for Electrode Materials of Rechargeable Batteries

The intrinsic limitations of lithium-ion batteries (LIBs) with regard to safety, cost, and the availability of raw materials have promoted research on so-called "post-LIBs". The recent intense research of post-LIBs provides an invaluable lesson that existing electrode materials used in LIBs may not perform as well in post-LIBs, calling for new material designs compliant with emerging batteries based on new chemistries. One promising approach in this direction is the development of materials with intercalated water or organic molecules, as these materials demonstrate superior electrochemical performance in emerging battery systems. The enlarged ionic channel dimensions and effective shielding of the electrostatic interaction between carrier ions and the lattice host are the origins of the observed electrochemical performance. Moreover, these intercalants serve as interlayer pillars to sustain the framework for prolonged cycles. Representative examples of such intercalated materials applied to batteries based on Li⁺ , Na⁺ , Mg²⁺ , and Zn²⁺ ions and supercapacitors are considered, along with their impact in materials research.

Atomistic Insights into FeF3 Nanosheet: An Ultrahigh-Rate and Long-Life Cathode Material for Li-Ion Batteries

Iron fluoride with high operating voltage and theoretical energy density has been proposed as a high-performance cathode material for Li-ion batteries. However, the inertness of pristine bulk FeF₃ results in poor Li kinetics and cycling life. Developing nanosheet-based electrode materials is a feasible strategy to solve these problems. Herein, on the basis of first-principles calculations, first the stability of FeF₃ (012) nanosheet with different atomic terminations under different environmental conditions was systematically studied, then the Li-ion adsorption and diffusion kinetics were thoroughly probed, and finally the voltages for different Li concentrations were given. We found that F-terminated nanosheet is energetically favorable in a wide range of chemical potential, which provide a vehicle for lithium ion diffusion. Our Li-ion adsorption and diffusion kinetics study revealed that (1) the formation of Li dimer is the most preferred, (2) the Li diffusion energy barrier of Li dimer is lower than isolated Li atom (0.17 eV for Li dimer vs 0.22 eV for Li atom), and (3) the diffusion coefficient of Li is 1.06 × 10^-6 cm²·s^-1, which is orders of magnitude greater than that of Li diffusion in bulk FeF₃ (10^-13-10^-11 cm²·s^-1). Thus, FeF₃ nanosheet can act as an ultrahigh-rate cathode material for Li-ion batteries. More importantly, the calculated voltage and specific capacity of Li on the FeF₃ (012) nanosheet demonstrate that it has a much more stable voltage profile than bulk FeF₃ for a wide range of Li concentration. So, few layers FeF₃ nanosheet provides the desired long-life energy density in Li-ion batteries. These above findings in the current study shed new light on the design of ultrahigh-rate and long-life FeF₃ cathode material for Li-ion batteries.

Building an Electronic Bridge via Ag Decoration To Enhance Kinetics of Iron Fluoride Cathode in Lithium-Ion Batteries

As a typical multielectron cathode material for lithium-ion batteries, iron fluoride (FeF₃) and its analogues suffer from poor electronic conductivity and low actual specific capacity. Herein, we introduce Ag nanoparticles by silver mirror reaction into the FeF₃·0.33H₂O cathode to build the electronic bridge between the solid (active materials) and liquid (electrolyte) interface. The crystal structures of as-prepared samples are characterized by X-ray diffraction and Rietveld refinement. Moreover, the density of states of FeF₃·0.33H₂O and FeF₃·0.33H₂O/Ag (Ag-decorated FeF₃·0.33H₂O) samples are calculated using the first principle density functional theory. The FeF₃·0.33H₂O/Ag cathodes exhibit significant enhancements on the electrochemical performance in terms of the cycle performance and rate capability, especially for the Ag-decorated amount of 5%. It achieves an initial capacity of 168.2 mA h g^-1 and retains a discharge capacity of 128.4 mA h g^-1 after 50 cycles in the voltage range of 2.0-4.5 V. It demonstrates that Ag decoration can reduce the band gap, improve electronic conductivity, and elevate intercalation/deintercalation kinetics.

Amorphous FeF3/C nanocomposite cathode derived from metal–organic frameworks for sodium ion batteries

A cathode of FeF₃/C nanocomposites has been fabricated by a simple vapor-solid fluoridation route which shows superior Na-ion storage performance.

Ammonium Fluoride Mediated Synthesis of Anhydrous Metal Fluoride-Mesoporous Carbon Nanocomposites for High-Performance Lithium Ion Battery Cathodes

Metal fluorides (MF_x) are one of the most attractive cathode candidates for Li ion batteries (LIBs) due to their high conversion potentials with large capacities. However, only a limited number of synthetic methods, generally involving highly toxic or inaccessible reagents, currently exist, which has made it difficult to produce well-designed nanostructures suitable for cathodes; consequently, harnessing their potential cathodic properties has been a challenge. Herein, we report a new bottom-up synthetic method utilizing ammonium fluoride (NH₄F) for the preparation of anhydrous MF_x (CuF₂, FeF₃, and CoF₂)/mesoporous carbon (MSU-F-C) nanocomposites, whereby a series of metal precursor nanoparticles preconfined in mesoporous carbon were readily converted to anhydrous MF_x through simple heat treatment with NH₄F under solventless conditions. We demonstrate the versatility, lower toxicity, and efficiency of this synthetic method and, using XRD analysis, propose a mechanism for the reaction. All MF_x/MSU-F-C prepared in this study exhibited superior electrochemical performances, through conversion reactions, as the cathode for LIBs. In particular, FeF₃/MSU-F-C maintained a capacity of 650 mAh g^-1_FeF3 across 50 cycles, which is ∼90% of its initial capacity. We expect that this facile synthesis method will trigger further research into the development of various nanostructured MF_x for use in energy storage and other applications.

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

Monodispersed mesoporous iron fluorides were synthesized by a low-cost reversed micelle method. The as-prepared materials with hierarchical mesoporous structure exhibit excellent rate capability (115.6 mAh g^-1 at 2000 mA g^-1) which is superior to many other carbon-free iron fluorides. In addition, a high reversible capacity of 143.2 mAh g^-1 can be retained after 100 cycles at 1000 mA g^-1. The outstanding electrochemical features can be attributed to the particular hierarchical mesoporous structure, facilitating electrolyte penetration as well as rapid electronic and ionic transportation.

A first-principle study of the effect of OH− doping on the elastic constants and electronic structure of HTB-FeF3

FeF₃ with a hexagonal-tungsten-bronze structure (HTB-FeF₃) belongs to one type of promising cathode material for Li-ion batteries. HTB-FeF_3−x(OH)x (x = 0.083, 0.167, 0.333, 0.667, 0.833) can stably exist in usual growth conditions.