Lithium metal batteries based on Li metal anode coupled with conversion-type cathode have emerged to meet the demands of next-generation energy storage technology for large-scale application of powerful electromobility systems. Among conversion-type cathodes, iron trifluoride (FeF₃) is considered as a promising candidate which can offer an extremely high energy density of 1947 Wh/kg via three-electron transfer. Different from molecule cathodes, it can effectively mitigate the loss of cathode active species and the occurrence of anode side reactions caused by the difficulty of reaction-zone confinement.  

However, the intrinsic solid-solid conversion of fluoride is sluggish during repeated splitting and rebonding of metal-fluorine moieties. Especially in Li-driven fluoride conversion, heterogeneous precipitation and coverage of insulating lithium fluoride (LiF) on the whole electrode surface would impede the internal chemical reaction between active fluoride and lithium, causing large voltage hysteresis and low available capacity. 

Recently, the research team led by Prof. LI Chilin from Shanghai Institute of Ceramics of the Chinese Academy of Sciences made progress in conversion-type lithium-fluoride batteries. The findings were published in Science Advances. 

In view of the sluggish kinetics and poor reversibility of lithium-fluorine conversion reactions, the researchers proposed a novel solid-liquid fluorine conversion mechanism enabled by a fluoride anion receptor of tris(pentafluorophenyl)borane (TPFPB).  

TPFPB promotes the dissociation of inert lithium fluoride and provides a facile “fluorine transport channel” at multiphase interfaces via the formation of solvated F^- intermediate therein. The construction of solid-liquid channel could bypass tough solid-solid conversion and upgrade the fluoride conversion kinetics, and then achieve the sustaining cycling of conversion-type lithium-fluoride batteries with high capacity and energy efficiency.  

Besides, TPFPB has an electron deficient boron center which exhibits a strong attraction to electron-rich fluorine. TPFPB with F-binding affinity favorably promotes LiF splitting, and enables the F-state transformation from the solid LiF lattice to the solvated [TPFPB-F]^- intermediate. For the Li-driven Fe-F conversion system, the dissolved F^- tends to react with the oxidized Fe species in the reconversion process. The construction of solid-liquid fluorine channel not only improves the original rough solid-solid contact, but also kinetically promotes the F-sublattice conversion from Li-F to Fe-F structure. 

For the structure optimization of fluoride cathode, the researchers developed a thermal-induced self–oxygen penetration method. Two kinds of iron (oxy)fluoride composites (denoted as FeO_0.3F_1.7 and FeO_0.7F_1.3) were synthesized via sequential hydroxylation/dehydroxylation of hydrated iron fluoride under annealing treatment. The O doping in fluoride regulates the phase evolution pathway and introduces a stable second-generation parent phase of rock salt in the confined voltage region for conversion reaction. 

Benefiting from the construction of facile round-trip F/Li-transport pathways and the optimization of fluoride structure, FeO_0.3F_1.7 and FeO_0.7F_1.3 cathodes enable the sustaining conversion reaction with energy efficiency approaching 80%, high capacity retention of 472 and 484 mAh/g after 100 cycles, and superior rate capability with reversible capacities of 271 and 320 mAh/g at 2 A/g. Their energy densities are achieved at 1100 Wh/kg for FeO_0.3F_1.7 and 700 Wh/kg for FeO_0.7F_1.3 under the power densities of 220 and 4300 W/kg, respectively.  

The key finding of solid-liquid fluorine channel provides an effective strategy to develop fluorine-conversion battery systems with high energy density.