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.

Fig. 1. Reversible topochemical fluoridation of Bi _1.6 Pb _0.4 PtO ₄ and Bi ₂ PdO ₄ . (A) Reaction scheme for topochemical fluoride-ion insertion in Bi_1.6Pb_0.4PtO₄ and Bi₂PdO₄. (B) Expanded view of fluoridated Bi_1.6Pb_0.4O₄ and Bi₂PdO₄ along the c-axis indicating F-ion positions along the 1D tunnel defined by Bi lone-pair repulsions; and (C) a view down the crystallographic axis, showing chains of edge-sharing square planar PtO₄/PdO₄ units sharing two oxygens with BiO₆ octahedra that are asymmetrically distorted owing to the stereochemically expression of Bi³⁺ 6s² electrons. The PtO₄/PdO₄ and BiO₆ structural motifs separate the redox site from the F-ion binding site. Bi adopts a capped trigonal prismatic electron geometry with one vertex occupied by an electron lone pair (see Fig. S1†). Two long Bi–O (2.803 Å) bonds are weakly coordinated, and as such, have been removed to highlight the 1D tunnels with interstitial sites that accommodate F-ions.

Fig. 2. Structure evolution of Bi _1.6 Pb _0.4 PtO ₄ and Bi ₂ PdO ₄ upon topochemical fluoride-ion insertion. (A) Comparison of powder XRD patterns collected for Bi_1.6Pb_0.4PtO₄, Bi_1.6Pb_0.4PtO₄F_x obtained by treatment with XeF₂ in acetonitrile, and Bi_1.6Pb_0.4PtO₄ recovered after treatment with n-butyllithium. The reflections observed around 2θ = 26° (denoted with *) for Bi_1.6Pb_0.4PtO₄F_x were similar to that observed by Kageyama et al. and can be ascribed to interphasic products (e.g. PtF₂, PtF₄, BiF₃, BiOF). The reflection at 2θ = 43° (denoted with ′) for Bi_1.6Pb_0.4PtO₄_n-BuLi is further likely an interphasic lithiated species. (B) Expanded view showing evolution of the (211) and (220) reflections across the F-ion insertion and deinsertion reactions. Normalized k³-weighted Fourier-transformed R-space data extracted from (C) Pt L_III-edge EXAFS spectra for Bi_1.6Pb_0.4PtO₄ and Bi_1.6Pb_0.4PtO₄F_x; and (D) Pd K-edge EXAFS spectra for Bi₂PdO₄, and Bi₂PdO₄F_x. Comparison of normalized (E) Pt L_III-edge XANES spectra for Bi_1.6Pb_0.4PtO₄ and Bi_1.6Pb_0.4PtO₄F_x; and (F) Pd K-edge XANES spectra for Bi₂PdO₄ and Bi₂PdO₄F_x.

Fig. 3. HAXPES and magnetic measurements to probe evolution of electronic structure upon topochemical fluoridation. High-resolution core-level HAXPES plots at 2 keV incident energy for (A) Pt 4f, (B) Pd 3d, and Bi 4d core excitations collected for (C) Bi_1.6Pb_0.4PtO₄, Bi_1.6Pb_0.4PtO₄F_x, and Bi_1.6Pb_0.4PtO₄ recovered after treatment with n-butyllithium and (D) Bi₂PdO₄, Bi₂PdO₄F_x, and Bi₂PdO₄ recovered after treatment with n-butyllithium. FC magnetic susceptibility as a function of temperature for (E) Bi_1.6Pb_0.4PtO₄/Bi_1.6Pb_0.4PtO₄F_x and (F) Bi₂PdO₄/Bi₂PdO₄F_x under an applied field of 0.1T.

Fig. 4. Evolution of the electronic structure of Bi _1.6 Pb _0.4 PtO ₄ and Bi ₂ PdO ₄ upon F-ion insertion, examined using valence-band HAXPES and COHP analysis. Overlay of valence-band HAXPES spectra collected at incident photon energies of 2.0 keV and 5.0 keV for (A) Bi_1.6Pb_0.4PtO₄ (Pb 6s² states also contribute to bonding (B) and anti-bonding (AB) interactions) and (B) Bi₂PdO₄. Differentials in spectral weight as a function of incident photon energy are shaded in light blue. High-resolution HAXPES data collected for Bi_1.6Pb_0.4PtO₄ and Bi_1.6Pb_0.4PtO₄F_x at incident photon energy of (C) 2 keV and (D) 5 keV. All valence-band HAXPES spectra have been normalized to internal core level Bi 5d_5/2 peak at a binding energy of 24 eV near the valence band. COHP analysis of (E) Pt–O and (F) Bi–O interactions.

Fig. 5. Charge distribution and molecular orbital perspective of topochemical fluoride-ion insertion. Electron localization function (ELF) map of (A) Bi_1.6Pb_0.4PtO₄, and (B) Bi_1.6Pb_0.4PtO₄F_x. A 2-D cross-section of the 3-D ELF map shown in (C) A and (D) B. (E) Charge density difference (CDD) of Bi_1.6Pb_0.4PtO₄ with a fluoride ion inserted. (F) Charge redistribution around the oxidized Pt atoms. (G) Molecular orbital sketch showing the relative position of Bi 6s, 6p–O 2p, and Bi 6s, 6p–F 2p lone pair states relative to the Pt 5d–O 2p AB states.