Publication date: 10th April 2024
Materials containing ns2 lone pair demonstrate exceptional fluoride-ion conductivity[1], making them promising solid electrolyte candidates for all-solid-state fluoride-ion batteries. However, the influence of lone pairs, particularly 6s2 in Pb2+, on fluoride-ion conduction is not well understood. In this work, we explored the relationship between ionic conductivity, crystal structure, and electronic structure in CsPb0.9K0.1F2.9.[2] The cubic CsPb0.9K0.1F2.9 showed a remarkably low activation energy of 7.9 kJ mol–1, leading to high conductivity (1.0×10–3 S cm–1, bulk) at 223 K. Local migration of fluoride ions with a low activation barrier of 3.8 kJ mol–1 was verified by 19F nuclear magnetic resonance spectroscopy. NEB calculations revealed an 8 kJ mol–1 migration energy for CsPbF3. As a comparison, the fluoride ion migration barrier for CsSrF3 was calculated, which exhibited a relatively high value of 35 kJ mol–1, emphasizing the impact of ns2 electrons in B-site cations on reducing the migration barrier. Electron density analysis indicated that fluoride ions migrate through an exchange reaction involving Pb 6s lone pairs. Pb 6s is asymmetric and localized towards the F vacancy in PbF5 square pyramids, while it is symmetric and delocalized in PbF6 octahedra. During the transition state, a fluoride ion cleaves the Pb-F bond, creating a new vacancy and localizing the Pb electron density. The site-projected PDOS shows that the DOS of Pb shifts to the lower-energy side in PbF5 polyhedra, stabilizing the geometry and reducing the migration barrier of fluoride ions. These findings are beneficial for material design, providing superionic conductivity with a low migration barrier for fluoride ions as well as other anions.
This study was conducted using grants from two projects (JPNP16001 and JPNP21006) commissioned by the New Energy and Industrial Technology Development Organization (NEDO).
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