Insights into the Effects of Electrolyte Composition on the Performance and Stability of FeF₂ Conversion‐Type Cathodes

Abstract

As an alternative to commercial Ni-and Co-based intercalation-type cathode materials, conversion-type metal fluoride (MF x ) cathodes are attracting more interest due to their promisses to increase cell-level energy density when coupled with lithium (Li) or silicon (Si)-based anodes. Among metal fluorides, iron fluorides (FeF 2 and FeF 3 ) are regarded as some of the most promising candidates due to their high capacity, moderately high potential and very low cost of Fe. In this study, we systematically investigated the impacts of electrolyte composition on the performance and stability of nanostructured FeF 2 cathodes.We discovered dramatic impacts of Li salt composition, Li salt concentration, solvent composition and cycling potential range on the cathode most critical performance parameters -stability, capacity, rate and voltage hysteresis. In contrast to previous beliefs, we observed that even if the Fe 2+ cation dissolution could be avoided, the dissolution of Fanions may still negatively affect cathode performance. Formation of the more favorable cathode solid electrolyte interface (CEI) was found to minimize both processes.

Understanding the conversion mechanism and performance of monodisperse FeF2 nanocrystal cathodes

“…This stability is remarkable, even compared to the best electrolyte systems used in the literature, and stands in stark contrast to the rapidly increasing charge transfer resistance observed in the LP30 electrolyte. 24 In LP30 the charge transfer resistance fluctuates significantly between discharge and charge (Figure 2C). This is possibly related to the repetitive formation and partial dissolution of the SEI layer as corroborated by x-ray photoelectron spectroscopy (XPS).…”

“…Huang et al showed that electrolyte selection could greatly affect the performance of FeF 2 electrodes. 24 In this work, 1 molal lithium bis(fluorosulfonylimide) (LiFSI) dissolved in the N-methyl-N-propyl pyrrolidinium FSI (Pyr 1,3 FSI) ionic liquid is presented as a superior electrolyte for the cycling of transition metal fluorides. The ionic liquid electrolyte precludes major failure mechanisms, preserves the nanorod morphology during cycling, and obviates the use of "strongly coupled" carbon frameworks.…”

Novel K2Ti8O17 Anode via Na+/Al3+ Co-Intercalation Mechanism for Rechargeable Aqueous Al-Ion Battery with Superior Rate Capability

“…Noticeably, a slightly larger discharge capacity is observed in the first 10 cycles and then decreases. Irreversible changes likely occurred in the electrode at the early stage [ 33 , 34 ]. Furthermore, the as-assembled AIB has the power to light-up a commercial light-emitting diode (LED) lasting more than one hour ( Figure 6 h), confirming its promising practical applications for energy storage.…”

Dendrite-structured FeF2 consisting of closely linked nanoparticles as cathode for high-performance lithium-ion capacitors

“…To address these problems, great efforts have been made focusing on the construction of nanostructured FeF 2 . For example, Zhang et al reported FeF 2 /carbon composite nanoparticles with a size of 50-100 nm that showed a capacity of 183 mA h g À1 at 60 mA g À1 [34], while Huang et al realized a capacity of 300 mA h g À1 at 140 mA g À1 by fabricating nanostructured FeF 2 of the sub 50 nm size [35]. Therefore, desirable electrochemical performance is expected to be achieved for FeF 2 materials through rational design and modification [28,36].…”