Revisiting metal fluorides as lithium-ion battery cathodes

Abstract

Metal fluorides, promising lithium-ion battery cathode materials, have been classified as conversion materials, due to the reconstructive phase transitions widely presumed to occur upon lithiation. We challenge this view by studying FeF3 using X-ray total scattering and electron diffraction techniques that measure structure over multiple length-scales coupled with DFT calculations, and by revisiting prior experimental studies of FeF2 and CuF2. Metal fluoride lithiation is instead dominated by diffusioncontrolled displacement mechanisms, a clear topological relationship between the metal fluoride Fsublattices and that of LiF being established. Initial lithiation of FeF3 forms FeF2 on the particle's surface, along with a cation-and stacking-disordered phase, A-LixFeyF3 -structurally related to α-/β-LiMn 2+ Fe 3+ F6, which topotactically transforms to Band then C-LixFeyF3, before forming LiF and Fe. Lithiation of FeF2 and CuF2 results in a buffer phase between FeF2/CuF2 and LiF. The resulting principles will aid future developments of a wider range of isomorphic metal fluorides.

Construction of solid-liquid fluorine transport channel to enable highly reversible conversion cathodes

“…The interfacial charge transfer in the following conversion reaction certainly contributes to the higher-fraction diffusion-controlled processes, as indicated by the lowering of b value (around 0.7 to 0.8). The nanoscale effect and topological relationship between bulk F sublattices of metal fluoride and LiF do not cause a notable decrease of b values (53,54). The ratio of capacitive contribution can be quantitatively evaluated according to the equation i(V) = k 1  + k 2  1/2 , where the i value is the combinational contributions from capacitance effect (k 1 ) and intercalation process (k 2  1/2 ).…”

Extracting interface correlations from the pair distribution function of composite materials

“…operando total scattering measurements. For example, the intermediate “Li 2 FeF 4 ” phase discovered in our recent study of FeF 2 lithiation 36 might be equally well interpreted as the LiF∥FeF 2 interface. No doubt there will be many other such examples.…”

“…In this study, we show that this interface contribution to the PDF (we call it the 'iPDF') can actually be extracted using non-negative matrix factorisation (NMF) analysis 34 -a method only recently applied to PDF data as a means of deconvolving the function into its individual component factors. [35][36][37] We begin by establishing proof-of-concept, using idealised PDF data generated from suitably-constructed models of the Fe∥Fe 3 O 4 interface. We then report experimental X-ray total scattering measurements for controlled corrosion of polycrystalline Fe powder.…”

Morphology control and interface characteristics of well-dispersed nanomaterials in K-ion batteries

“…Recently, 3d-transition-metal binary uorides have been extensively investigated as potential electrode materials. 1,2 The idea to exploit uorides as batteries arose from the intrinsic stability of uorinated materials and their ability to generate high levels of electrochemical energy. However, in LIBs and sodium-ion batteries (NIBs), transition metal binary uorides could not be used as cathodes against a pristine carbonaceous anode, because of the lack of lithium or sodium for the current carrier.…”