Weberite Na2MM′F7 (M, M′ = redox-active metal) as promising fluoride-based sodium-ion battery cathodes

Abstract

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 Na₂MM′F₇ (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 mA h g⁻¹), and low net Na-ion migration barriers (<600 meV). Significantly, ten out of the twelve screened compounds exhibit high specific energies exceeding 580 W h kg⁻¹ (approximately equal to the specific energy of LiFePO₄), indicating their exceptional electrochemical performance. This study will pave the way for further advancements in fluoride-based electrode materials.