Open Framework Fluorides As Insertion/Conversion Cathodes for Li and Na Batteries
Authors: Li Chilin 
Published: February 2020

Recently, low-density porous framework solids, such as materials based on metal-organic frameworks (MOFs), zeolitic frameworks, or Prussian blue (PB) analogues, have been attracting significant attention because of their diverse applications as energy storage materials, proton conductors, wide-band-gap semiconductors, catalysts, selective gas absorbers, and nanoporous carbon precursors. These phases are generally assembled at low temperature by precipitation, solvothermal, or ionothermal synthesis. Ionic liquids (ILs) are well-suited candidates as reaction media, with the cations potentially serving as structure-directing templates to order and polymerize surrounding inorganic/organic units, likely leading to structural expansion with lowered crystal density. This strategy was recently discovered to be especially useful for developing novel open-structure Li storage cathodes (e.g., tavorite polyanion frameworks or iron fluoride polymorphs [1-4]) that are characterized by enhanced ion transport in multidimensional channels and extended solid-solution reaction zones.
Electroactive MOFs and PB analogues exhibit quite open channels, the sizes of which may even reach the mesoporous range and hence exceed the optimal width for ion insertion. Furthermore, these frameworks are usually of high molecular weight because of their heavy skeletons and/or the existence of channel fillers. As far as electrochemical storage is concerned, these factors limit the reversible capacity in the context of Li batteries. In terms of electrochemical storage, not only are Li-based batteries of interest, but also in view of globally limited Li resources, Na-based batteries are seen as a potential alternative for both electric vehicles and large-scale grid-based electrical energy storage. As the ionic radius of Na+ (1.02 Å) is 34% larger than that of Li+(0.76 Å), suitable Li storage materials are not automatically feasible for Na batteries, and in turn, channels that are oversized for Li may be well-suited for Na. To further improve the energy/power density of Li- or Na-based batteries, structure modification of oxide/polyanion materials with moderate expansion of ion channels has been demonstrated to be a successful strategy. However, one should pay attention to the fact that the synthesis of more expanded structures usually comes at the cost of a decrease in ion channel dimensionality, mostly leading to the formation of 1D or 2D channel structures that have a greater potential to become blocked or degraded. Also, in the case of interconnected open 3D channels, structure expansion upon storage is still a great challenge.
Fluorides are expected to exhibit larger capacities than most of the oxides without tradeoffs in terms of working voltage as long as their poor conductivity can be compensated. In the past decades, such studies mainly focused on commercially available ReO3-type FeF3 electroactivated by high-energy ball milling of FeF3/C composites to generate C-FeF3 nanodomains as insertion or conversion cathodes for Li batteries. Recently, our group prepared an open-structure fluoride for Li battery applications.[2,5] As reported for the hexagonal tungsten−bronze (HTB)-type compound FeF3·0.33H2O characterized by open 1D channels, the Li insertion mechanism was intrinsically modified, and the miscibility gap present in ReO3-type FeF3 was completely removed in the HTB phase, favoring complete solid-solution behavior in the 3 V region. An improved intrinsic conductivity enabled the fluoride to act as a highly electroactive Li battery cathode without in situ addition of conductive species. However, the presence of single ion channels, which are prone to partial blockage by H2O molecule fillers, still limits the extension of fluoride materials into Na batteries.
Most recently, we report a novel open-framework fluoride pyrochlore phase, FeF3·0.5H2O, that is structurally similar to the known AlF3·0.5H2O and characterized by a much larger cell volume (∼1130 Å3) than in HTB-type FeF3·0.33H2O (∼710 Å3) or ReO3-type FeF3 (∼310 Å3). [6] Notably, it exhibits a higher pore density due to interconnected 3D ion channels without a serious tradeoff concerning channel size compared with HTB-type fluoride, suggesting a more favorable cation insertion capacity. The storage performance should also benefit from the more tightly confined H2O molecules in the zigzag channels of the pyrochlore phase as opposed to the straight channels of the HTB phase.

References

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