Lithium-ion batteries (LIBs) have dominated the market of portable electronics and are penetrating that of electric vehicles. Currently, commercial LIBs adopt a rocking chair-type reaction to store energy – lithium ions intercalate/deintercalate in a rigid host (cathode) such as LiCoO₂, LiFePO₄, and LiNi_xMn_yCo_{1 – x – y}O₂. The space within these hosts limits the capacity, thereby constraining their energy density. Metal fluorides, on the other hand, is a new type of cathode which goes through a structural change during lithiation. Therefore, its capacity is no longer dependent on its internal void space. These materials are capable of store large amount of lithium and therefore are promising cathodes for next-gen LIBs with much higher energy densities. However, the structural changes of these metal fluorides during lithium insertion are complicated and understanding such is bottlenecking further development of these fluoride towards a commercial stand. 

A collaborative research between scientists from University of Cambridge, University of Manchester, University of Oxford, Argonne National Laboratory, Rutgers University, and Adolphe Merkle Institute revealed the structural changes of metal fluorides during lithiation to its finest detail. By combining a series of cutting-edge characterization techniques including electron diffraction, pair distribution function analysis, density functional theory calculations, and ab initio crystal structure predictions, researchers found that the lithium storage in metal fluorides is dominated by a diffusion-controlled displacement reaction during which the F⁻ sublattice topologically links the reactants and the reaction products. Therefore, the diffusion of displaced species bottlenecks the reaction kinetics and is the key to the future success of metal fluorides as cathodes for next-gen batteries. The results are reported in the journal Nature Materials. 

Figure: Structural changes of FeF₂ metal fluoride cathodes during lithiation.