Exploiting the Iron Difluoride Electrochemistry by Constructing Hierarchical Electron Pathways and Cathode Electrolyte Interface

Abstract

Conversion‐type cathodes such as metal fluorides, especially FeF2 and FeF3, are potential candidates to replace intercalation cathodes for the next generation of lithium ion batteries. However, the application of iron fluorides is impeded by their poor electronic conductivity, iron/fluorine dissolution, and unstable cathode electrolyte interfaces (CEIs). A facile route to fabricate a mechanical strong electrode with hierarchical electron pathways for FeF2 nanoparticles is reported here. The FeF2/Li cell demonstrates remarkable cycle performances with a capacity of 300 mAh g−1 after a record long 4500 cycles at 1C. Meanwhile, a record stable high area capacity of over 6 mAh cm−2 is achieved. Furthermore, ultra‐high rate capabilities at 20C and 6C for electrodes with low and high mass loading, respectively, are attained. Advanced electron microscopy reveals the formation of stable CEIs. The results demonstrate that the construction of viable electronic connections and favorable CEIs are the key to boost the electrochemical performances of FeF2 cathode.

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

“…Typically, liquid-phase synthesis, including chemical precipitation, [60] hydrothermal, [61] solvothermal, [62,63] sol-gel, [64] and thermal decomposition methods [65] based on different fluorine sources, (hydrogen fluoride (HF) solution, [66] NH 4 F, [67] NH 4 HF 2 , [68,69] metal hexafluorosilicate, [70] F-containing ionic liquids (ILs), [71] etc.) has been utilized to prepare nanoparticle MFs, [72] metal oxyfluorides, [73] mixed metal difluorides, [74] and MF-carbon composites.…”

“…To improve the electrical conductivity, mechanical stability, and other performance metrics (such as kinetic behaviors, side reactions, and dissolution), various nanostructured materials, especially conductive materials with an electron transportation network around the active materials, have been explored. Carbon materials (carbon blacks, [25] graphene, [157][158][159] CNTs, [84,86] carbon fibers, [160] carbon cloth, [70] and various porous/nanopore carbons [161,162] ) are perfect for connecting or coating materials that are highly conductive and easily synthesized with controllable morphology and porosity. A summary of the different composite electrochemical performances is shown in Table 1.…”

FeF3@C nanotube arrays grown on carbon fabric as a free-standing cathode for lithium-ion batteries

“…[15][16][17][18] On account of this, many new types of cathodes, such as FeF 3 , FeF 3 Á0.33H 2 O, FeF 2 , FeOF, FeS 2 , Co 9 S 8 , and so on, have been discovered for Li-ion batteries by researchers. [19][20][21][22][23][24][25][26][27][28][29][30] Among them, FeF 3 , a typical new-type cathode, could accommodate three Li + and achieve an ultrahigh theoretical capacity of 712 mA h g À1 , which has attracted considerable attention from researchers. [31][32][33][34][35] Furthermore, iron is the second most abundant resource and occupies 5% of the earth.…”

Nanoparticulate FeF2@C as a Li Battery Conversion Cathode

“…Some of the most promising conversion cathode materials are the iron fluorides, FeF 2 and FeF 3 (collectively referred to as FeF x ), with theoretical capacities of 571 and 712 mAh/g, respectively. FeF 3 has a higher theoretical capacity than FeF 2 but also undergoes a more complex and partially irreversible lithiation/delithiation mechanism. , As a result, although it has a lower capacity, the more robust FeF 2 remains an exciting choice for further study. , Ultimately, challenges such as slow lithiation/delithiation kinetics and material stability plague both FeF x materials and have been the focus of many studies. ,− Kinetic limitations have previously been addressed by specialized processing and synthesis, e.g., ball milling, and nanoparticle fabrication, , as well as specialized current collectors . In each case, the goal is to use smaller particles to circumvent the kinetic and conductivity limitations inherent to larger particles.…”

“…4,10−13 Kinetic limitations have previously been addressed by specialized processing and synthesis, e.g., ball milling, 14 and nanoparticle fabrication, 9,15 as well as specialized current collectors. 16 In each case, the goal is to use smaller particles to circumvent the kinetic and conductivity limitations inherent to larger particles. Attempts at improving material stability typically rely on protective surface coatings, applied both prior to cell fabrication using ex situ techniques, 17−19 and by careful choice of electrolyte to produce a protective cathode electrolyte interphase (CEI) in situ.…”