Design of Fluoride-Ion Battery Insertion Electrodes Based on Stereochemically Active Lone Pairs

Lithium-ion insertion batteries have revolutionized modern consumer electronics due to their unmatched power densities, yet their large-scale demand and production is giving rise to new concerns on materials criticality. One less explored method to advance energy storage technology while remaining environmentally cognizant is to utilize fluoride-ion batteries. While still in their nascent stage, certain design rules have emerged to help expedite the discovery of new electrodes capable of fluoride-ion insertion. For example, the host crystal structure should contain large tunnels with vacant interstitial positions and be composed of a redox-active transition metal and a p-block cation with stereochemically active lone pairs, which help facilitate anion diffusion. Unfortunately, these design rules have only been applied to materials that crystallize in the Schafarzikite type, which has hindered the development of new electrodes. This work aims to explore the generalizability of these design rules by investigating PbPdO₂ and Sn₂TiO₄ as new fluoride-ion insertion electrodes. PbPdO₂ and Sn₂TiO₄ were synthesized and fluoridated upon reaction with a molar excess of XeF₂, which was confirmed using X-ray absorption and variable X-ray emission spectroscopies and magnetism. Crystal orbital Hamilton population (COHP) calculations and the measurement of the valence band of PbPdO₂, PbPdO₂F<i>_x</i>, Sn₂TiO₄ and Sn₂TiO₄F<i>_x</i> using X-ray absorption spectroscopy yields a comprehensive bonding analysis to understand the mechanism of fluoridation within these materials. We show that host structures containing cations with a formal ns2np0 electronic configuration and stereochemical active lone pairs underpin the formation of large one-dimensional tunnels that contain interstitial positions for fluoride ion insertion and the interactions between the active lone pair electrons and fluoride facilitate reversible anion diffusion. This work verifies the applicability of these design rules to new structure types, which drastically expands the structural and compositional spaces of interest and can expedite the discovery of new electrodes capable of reversible, room-temperature fluoridation.