Designer Mixed-Anion Materials for Solar-Energy Harvesting and Fluoride-Ion Batteries

Mixing multiple cations has been a successful strategy to tailor the properties of functional ceramics, and even induce new properties. In contrast, mixing on the anion sub-lattice, or anion engineering, has remained largely unexplored. Given the key role anions play in the materials’ structure and property, and owing to the different elemental characteristics of anions, such as charge, ionic radii, electronegativity, and polarizability, anion engineering holds a lot of untapped potential to control the stability and functionality of materials. In this talk, we will present results on designer, anion engineered compounds that can efficiently harness solar energy as photovoltaics and photocatalysts, and enable high-capacity intercalation fluoride-ion batteries.<br/>In the first part of the talk, we will discuss results of a first-principles-based design of a new family of ferroelectric semiconductors with <i>A</i>³⁺SnO₂N (<i>A</i> = Y, Eu, La, In, and Sc) stoichiometry — that combine the strong binding of metal oxides and nitrides with the low carrier effective mass, and small, tunable band gaps of lead-halide perovskites [1]. These tin oxynitrides have predicted direct band gaps ranging from 1.6 to 3.3 eV and a sizable electric polarization up to 17 μC/cm², which is predicted to be switchable by an external electric field through a nonpolar phase. With their unique combination of polarization, low carrier effective mass, and band gaps spanning the entire visible spectrum, we expect <i>A</i>SnO₂N ferroelectric semiconductors can serve as photovoltaics and photocatalysts.<br/>In the last part of the talk, we will introduce layered electrides, such as Ca₂N and Y₂C – that have an electron occupying an anion lattice site – as promising hosts to act as fluoride intercalation anodes [2]. The fluoride ion is well suited to be the active species of rechargeable batteries, due to its small size, light weight, and high electronegativity. While existing F-ion batteries based on conversion chemistry suffer from rapid electrode degradation with cycling, those based on fluoride intercalation are currently less attractive then cation intercalation battery chemistries due to their low reversible energy densities. Using first-principles calculations, we predict that anodes made from layered electrides can offer voltage up to −2.86 V <i>vs.</i> La₂CoO₄ cathode, capacity &gt;250 mA h g⁻¹, and fast diffusion kinetics with migration barriers as low as 0.15 eV. These metrics compare favorably to popular Li-ion intercalation cathodes such as LiCoO₂. Therefore, electrides open up a new space for designing fluorine intercalation batteries with good performance and cyclability.<br/> Acknowledgments: This research was supported by NSF through DMR-1806147 <br/> <br/>[1] S. T. Hartman, A. S. Thind, and R. Mishra, Chemistry of Materials <b>32</b>, 9542 (2020).<br/>[2] S. T. Hartman and R. Mishra, Journal of Materials Chemistry A <b>8</b>, 24469 (2020).