Understanding the conversion mechanism and performance of monodisperse FeF2 nanocrystal cathodes

Abstract

The application of transition metal fluorides as energy dense cathode materials for lithium ion batteries has been hindered by inadequate understanding of their electrochemical capabilities/limitations. Here, we present an ideal system for mechanistic study through the colloidal synthesis of single crystalline, monodisperse iron(II) fluoride nanorods. Near theoretical capacity (570 mA h g −1 ) and extraordinary cycling stability (>90% capacity retention after 50 cycles at C/20) is achieved solely through the use of an ionic liquid electrolyte (1 m LiFSI/Pyr 1,3 FSI), which forms a stable solid electrolyte interphase and prevents the fusing of particles. This stability extends over 200 cycles at much higher rates (C/2) and temperatures (50 • C). High-resolution analytical transmission electron microscopy reveals intricate morphological features, lattice orientation relationships, and oxidation state changes that comprehensively describe the conversion mechanism. Phase evolution, diffusion kinetics and cell failure are critically influenced by surface specific reactions. The reversibility of the conversion reaction is governed by topotactic cation diffusion through an invariant lattice of fluoride anions and the nucleation of metallic particles on semi-coherent interfaces. This new understanding is used to showcase the inherently high discharge rate capability of FeF 2 .

A Coral‐Like FeP@NC Anode with Increasing Cycle Capacity for Sodium‐Ion and Lithium‐Ion Batteries Induced by Particle Refinement

“…Meanwhile,astable SEI film with constant-thickness can be observed during the whole discharge/charge process (Figure S16), which is favourable for achieving as table cycling performance. [19] Further,e xs itu Raman (Figure 3d)a nd ex situ Fourier transform infrared (FTIR) (Figure 3e)w ere used to capture the surface variation of the 6h-FeP@NC-1 during electrochemical processes by detecting the graphitization degree of carbon materials.T he I D /I G ratio decreased from 0.79 to 0.55 during the sodiation process,d emonstrating the enhanced graphitization degree, [6,20] which may be ascribed to more graphitized carbon layers.While the I D /I G rose to 0.80 during the desodiation process and it was close to the original value, indicating some special effect of the FePn anoparticles is…”

Artificial cathode solid electrolyte interphase to endow highly stable lithium storage of FeF2 nanocrystals

“…Another method is to stabilize the interface with Li salt composites [16,17]. Recently, Pasta's group [18] revealed that an in situ-formed cathode solid electrolyte interphase (CEI) layer could significantly improve the cycling stability of FeF 2 nanorods. However, the quality of the CEI layer strongly depends on the composition of the electrolyte and the working conditions.…”

Real Time Observation of Lithium Insertion into Pre-Cycled Conversion-Type Materials

“…Conversion-type electrode materials (MX, where M is generally a transition metal ion and X is O, S, F, etc.) have been extensively explored, as they can deliver much higher capacities than intercalation-type materials [1][2][3][4][5][6]. When lithium ions are introduced, MX is converted into M and Li-X via the conversion reaction.…”