Stereochemical expression of Bi 6s2 lone pairs mediates fluoride-ion (De)insertion in tunnel-structured Bi2PdO4 and Bi1.6Pb0.4PtO4

Abstract

Fluoride-ion batteries are a promising alternative to lithium-ion batteries by dint of the greater crustal abundance of fluorine and the potential to alleviate the need for metal electrodeposition. However, conventional metal fluoride cathodes typically rely on conversion-type reactions that require propagation of a reaction—diffusion front, thereby limiting cycling performance and rate capability. In contrast, the topochemical insertion of fluoride-ions in periodic solids remains a relatively unexplored approach. Here, we explore the mechanisms of fluoridation of Bi₂PdO₄ and Bi_1.6Pb_0.4PtO₄ insertion hosts that possess capacious tunnels that can accommodate fluoride-ions with a particular emphasis on elucidating the role of stereochemical expression of bismuth 6s² lone pairs in mediating anion diffusion. We reveal that the topochemical solution-phase insertion and deinsertion of fluoride-ions at room temperature is mediated by redox reactions at platinum and palladium centers but involves multi-center synergies between d- and p-block atoms across the one-dimensional (1D) tunnel structure. While Pt and Pd centers mediate redox reactions, the stereochemically active lone pair electrons of Bi³⁺ play a pivotal role in facilitating reversible fluoride-ion diffusion. Consequently, Bi_1.6Pb_0.4PtO₄ and Bi₂PdO₄ can be reversibly fluoridated with full recovery of the crystal lattice and with minimal alteration of the unit cell volume. The results reveal a key principle that the stereochemical activity of p-block electron lone pairs can be harnessed to modulate anion–lattice interactions and mediate facile anion diffusion.