Exploring electrides as anion storage materials

Many next-generation technologies rely on increasing the capacity of storage materials. Although lithium-ion batteries have attracted the most attention for energy storage, materials that store anions remain relatively unexplored. Anion shuttle batteries may offer high capacity and be made from abundant materials; however, all known anion battery electrodes are conversion-based, causing short lifetimes. We propose that electrides, exotic materials that have electrons present in well-defined lattice sites rather than within atoms, could be used as intercalation-type electrodes to fill this gap. Additionally, it is widely assumed that the gain or loss of electrons in a material must be accompanied by its reduction or oxidation. Here, we report a system in which the insertion/deinsertion of an electron occurs without any reduction or oxidation. Although all known electrides have an electropositive metal cation adjacent to the electride site, the effect of cation electronegativity on the properties of electrides is not yet known. Understanding these affects could lead to the discovery of novel electrides with higher capacities or more abundant elements than Y2C. Therefore, we examine trivalent metal carbides with varying degrees of electronegativity and experimentally synthesize Sc2C. Our studies identify the material as a two-dimensional (2D) electride, even though Sc is more electronegative than any metal previously found adjacent to an electride site. Further, by exploring Sc2C and Al2C computationally, we find that higher electronegativity of the cation drives greater hybridization between metal and electride orbitals, which opens a band gap in these materials. Sc2C is the first 2D electride semiconductor, and we propose a design rule that cation electronegativity drives the change in its band structure.High-capacity hydrogen storage materials may enable the use of hydrogen fuel cells in mobile applications; however, all known hydrogen storage materials are low capacity. Therefore, with the predicted high hydrogen storage capacity of 2D Sc2C, we experimentally explore the hydrogenation of Sc2C. We find that reacting Sc2C with H2 gas creates Sc2CHx, and that this process might be reversible at room temperature.