Na₂Fe₂F₇: a fluoride-based cathode for high power and long life Na-ion batteries

Abstract

T-Na2Fe2F7 based on three-dimensionally connected FeF6 octahedra exhibits large specific capacity and ultra-high-stable cycling performance as a promising cathode for NIBs.

Highly Stable Fe2+/Ti3+‐Based Fluoride Cathode Enabling Low‐Cost and High‐Performance Na‐Ion Batteries

“…The lattice strain was calculated using the Williamson–Hall isotropic strain model (W–H ISM) [ 50,51 ] based on the o ‐SXRD patterns, as previously studied using the same procedure. [ 20 ] As shown in Figures S15–S16 (Supporting Information), the slope of Na x TiFeF 7 (0 ≤ x ≤ 2), reflective of the lattice strain, negligibly increased from 0.00462 to 0.00493 (0.031%) during Na + deintercalation (from x = 2 to x = 0). Furthermore, it was reversibly recovered to 0.00469 (0.024%) after Na + intercalation (Figure S17, Supporting Information); moreover, the value was much less changed compared with Na 2 Fe 2 F 7 (≈0.249%).…”

“…Furthermore, the fluorine ions in the structure enable a higher redox potential of transition‐metal ions than oxygen ions through the inductive effect by the highest electronegativity. [ 19,20 ] However, development of fluoride‐based cathode material has been limited to a small number of candidates such as NaFeF 3 or KFeF 3 derivatives so far, and energy density is still incompetent compared to other existing Na cathodes. [ 21–24 ] Even, some of fluoride‐based cathodes require presodiation to fully utilize available high capacity.…”

Fluorinated Materials as Positive Electrodes for Li- and Na-Ion Batteries

“…For HTB and pyrochlore FeF 3 , these limits correspond x = 0.33 and x = 0.5 (as also shown by the maximum number of water molecules that can be accommodated in the HTB and pyrochlore structures); they transform to crystalline ReO 3 and amorphous phases, respectively . The recent studies of LiFe 2 F 6 trirutile and T-Na 2 Fe 2 F 7 weberite confirm that the thermodynamically stable phases are prone to give best electrochemical performances with high stability upon cycling. However, this is far to be a general rule.…”

“…But recently, a trigonal T-Na 2 Fe 2 F 7 ( P 3 1 21) prepared from a mixture of NaF, FeF 2 and FeF 3 at 650 °C under Ar showed, in NIB configuration, a high capacity close to the theoretical capacity of 134 mAh g –1 , without any phase transition and very low volume variation (Figure ). Over 88% of the initial capacity is still achievable after 1000 cycles . On the basis of this result, other cationic combinations have been tested in trigonal Na 2 MnVF 7 and in M-II monoclinic Na 2 MVF 7 types (M II = Fe, Co) giving poorer results, but still interesting with 100 mAh g –1 after 200 cycles (100 mA g –1 ) for monoclinic Na 2 FeVF 7 …”

“…Galvanostatic charge–discharge voltage profiles of the first cycles at different rates (left) and predicted 3D Na + diffusion pathways in the T-Na 2 Fe 2 F 7 structure (right) from Park et al Reproduced from ref . Copyright 2021 Royal Society of Chemistry.…”