  Sodium-ion batteries are a viable alternative to lithium-ion technology due
to the plentiful sodium resources. However, certain commercialization
challenges, such as low specific energies and poor cycling performance of
current Na-ion cathodes, still need to be addressed. To overcome these hurdles,
this study explored the potential of a novel class of fluoride-based materials,
specifically trigonal-type Na$_2$MM'F$_7$ (M and M' are redox-active metals)
belonging to the weberite-type compounds, as promising candidates for Na-ion
cathodes. Through a comprehensive assessment utilizing ab initio calculations,
twelve prospective compounds were identified, demonstrating high thermodynamic
stability, large gravimetric capacities (>170 mAh/g), and low net Na-ion
migration barriers (<600 meV). Significantly, ten out of the twelve screened
compounds exhibit high specific energies exceeding 580 Wh/kg (approximately
equals to the specific energy of LiFePO$_4$), indicating their exceptional
electrochemical performance. This study will pave the way for further
advancements in fluoride-based electrode materials.
